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Introduction

Subacute sclerosing panencephalitis (SSPE) is a late, rare and usually fatal complication of measles infection. Although a very high incidence of SSPE in Papua New Guinea (PNG) was first recognized 20 years ago, estimated measles vaccine coverage has remained at ≤70% since and a large measles epidemic occurred in 2002. We report a series of 22 SSPE cases presenting between November 2007 and July 2009 in Madang Province, PNG, including localized clusters with the highest ever reported annual incidence.

Methodology/Principal Findings

As part of a prospective observational study of severe childhood illness at Modilon Hospital, the provincial referral center, children presenting with evidence of meningo-encephalitis were assessed in detail including lumbar puncture in most cases. A diagnosis of SSPE was based on clinical features and presence of measles-specific IgG in cerebrospinal fluid and/or plasma. The estimated annual SSPE incidence in Madang province was 54/million population aged <20 years, but four sub-districts had an incidence >100/million/year. The distribution of year of birth of the 22 children with SSPE closely matched the reported annual measles incidence in PNG, including a peak in 2002.

Conclusions/Significance

SSPE follows measles infections in very young PNG children. Because PNG children have known low seroconversion rates to the first measles vaccine given at 6 months of age, efforts such as supplementary measles immunisation programs should continue in order to reduce the pool of non-immune people surrounding the youngest and most vulnerable members of PNG communities.

Author Summary

Subacute sclerosing panencephalitis (SSPE) is a disabling and usually fatal brain disorder that typically occurs 3–10 years after acute measles infection. Papua New Guinea (PNG) has particularly high rates of SSPE. We report 22 cases of PNG children presenting to the provincial referral hospital in Madang Province who probably contracted acute measles when <12 months of age during a national epidemic in 2002 and who developed SSPE 5–7 years later. Based on these cases, the estimated annual SSPE incidence in Madang province in 2007–2009 was 54/million population aged <20 years. Four sub-districts had an annual incidence >100/million population aged <20 years, the highest rates ever reported. Young PNG children do not respond well to measles vaccine. Because of this, efforts such as supplementary measles immunisation programs should continue in order to reduce the pool of non-immune older people surrounding the youngest and most vulnerable members of PNG communities.

The neurovirulence of two wild type (wt) and seven Subacute Sclerosing Panencephalitis (SSPE) measles virus strains was tested in young adult ferrets by intracerebral (IC) inoculation of infected Vero cell suspensions. Wt strains Edmonston and Woodfolk and SSPE strains Mantooth, Halle, and LEC-S did not produce a detectable encephalitis in the ferrets, but caused a significant formation of serum antibodies against measles virus. SSPE strains LEC, IP-3, Biken, and D.R., on the other hand, were all neurovirulent in ferrets, particularly strain D.R. which caused an acute encephalitis in all inoculated animals. Strain Biken was of particular interest since it caused a subacute encephalitis in four of seven ferrets. The subacute encephalitis was characterized by a long incubation time, persistence of virus in the brain for at least 8 mo, widespread inflammatory lesions, and production of measles virus specific IgG in the brain. A study of the biological properties of the various measles virus strains showed that wt strains Edmonston and Woodfolk and SSPE strains Mantooth, Halle, and LEC-S produced free virus particles in significant titers both in Vero and ferret brain (FB) cultures. Cytopathic effect (CPE) with cell- fusion was marked in Vero cultures, whereas only minimal CPE and no cell-fusion were observed in the FB cultures. SSPE strains LEC, IP-3, Biken, and D.R., on the other hand, were mostly cell-associated in Vero and FB cultures, although atypical cell-free particles were produced by strains Biken and IP-3. All four strains showed cell-fusing activity in FB cultures, particularly strain D.R., which was the only strain that spread more actively by fusion in FB than in Vero cultures. The results are discussed in relation to the neurovirulence of the various measles virus strains in adult ferrets. Pronounced cell-fusing activity in FB cells and cell-association with minimal or no production of cell-free virus seem to be essential to establish a brain infection in the animals.

A version of the Western blot was developed to detect serum antibodies against measles virus polypeptides. With this technique, a seroepidemiological survey of antibodies to the several measles virus proteins in diverse measles-related conditions was conducted. The sera were obtained from individuals with a recent or long-past history of natural measles, from persons with a history of immunization with live attenuated measles vaccine, and from patients with multiple sclerosis, subacute sclerosing panencephalitis, or atypical measles. The findings indicated that live attenuated measles vaccine elicits an antibody response qualitatively resembling that of a natural infection. In addition, multiple sclerosis patients made less antibody to the measles virus M protein than did individuals with a long-past history of natural measles. Thus, the immunological reaction of multiple sclerosis patients to measles virus is qualitatively, as well as quantitatively, different from that of normal persons. Finally, persons with subacute sclerosing panencephalitis and atypical measles mounted abnormally high antibody responses to measles virus polypeptides, in particular the P protein.

Between 1971 and 1989 measles encephalitis was identified in five children receiving chemotherapy for acute lymphoblastic leukaemia. Review of these and previously reported cases of measles encephalitis in immunosuppressed patients failed to identify any pathognomonic features in the history, the clinical presentation, or the results of electroencephalography or computed tomography. Detection of measles virus antigen in nasopharyngeal secretions or intrathecal synthesis of specific antibody was not possible in all instances. Early diagnosis by direct detection of viral antigen in the brain was confounded by difficulties in identifying areas of the brain suitable for biopsy. Increasing herd immunity to measles in the general population by vaccination is the only effective intervention against measles encephalitis in immunosuppressed children. Measles encephalitis must be remembered as a possible explanation of encephalopathy in the immunocompromised child: the benefits of early use of antiviral agents need to be evaluated.

Measles virus protein synthesis has been analyzed in acutely and persistently infected cells. To assess the role of measles in subacute sclerosing panencephalitis (SSPE), measles viral proteins synthesized in vivo or in vitro were tested for reactivity with serum from a guinea pig(s) immunized with measles virus and sera from patients with SSPE. Guinea pig antimeasles virus serum immunoprecipitates the viral polypeptides of 78,000 molecular weight (glycosylated [G]), 70,000 molecular weight (phosphorylated [P]), 60,000 molecular weight (nucleocapsid [N]), and 35,000 molecular weight (matrix [M]) from cells acutely infected with measles virus as well as from chronically infected cells, but in the latter case, immunoprecipitated M protein has a reduced electrophoretic migration. Sera of SSPE patients immunoprecipitated all but the G protein in acutely infected cells and only the P and N proteins from chronically infected cells. In immunoprecipitates of viral polypeptides synthesized in a reticulocyte cell-free translation system, in response to mRNA from acutely or persistently infected cells, the 78,000-molecular-weight form of the G protein was not detected among the cell-free products of either mRNA. Guinea pig antimeasles virus serum immunoprecipitated P, N, and M polypeptides from the products of either form of mRNA, whereas SSPE serum immunoprecipitated the P and N polypeptides but not the M polypeptide. The differences in immunoreactivity of the antimeasles virus antiserum and the SSPE serum are discussed in terms of possible modifications of measles virus proteins in SSPE.

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Subacute sclerosing panencephalitis is a rare disorder of late childhood and early adolescence Affected patients usually show behavioural and intellectual disturbance and involuntary movements before dying in coma after about 12 months. At some stage most have characteristic electroencephalographic abnormalities. Pathologically, changes in the brain are those of subacute encephalitis with a variable gliosis of the white matter, and sometimes intranuclear inclusion bodies in neurones and glial cells.

Recent studies in many patients have shown high levels of circulating anti-measles antibodies, measles antigen in cells in the brain, and sometimes, myxo-virus filaments in cells there. These findings suggest that SSPE may be a slow measles virus infection of the nervous system. Possible explanations for the slow evolution of the encephalitis include disordered immune mechanisms and intracellular persistence of virus in a defective phase.

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The occurrence of antibodies to the nucleoprotein and matrix (M) antigens of measles virus was determined in early and late measles convalescent sera and in sera from patients with multiple sclerosis, subacute sclerosing panencephalitis, chronic active hepatitis, and atypical measles. Antibodies to the two components were identified separately in serially diluted samples both by radioimmune precipitation assays and by complement fixation tests employing purified nucleoprotein and M components as antigens. The antibody response to M antigen in connection with acute infections was weak, and with time titers of antibodies to M antigen were reduced below detectable levels in most cases. A different situation was seen in patients with atypical measles. A pronounced antibody response to M antigen was shown to be a part of the generally accentuated immune response in these patients. Confirming results of others, it was shown that in spite of the increased antibody titers against most measles components in sera from patients with subacute sclerosing panencephalitis, no or only low titers of antibodies to M antigen were present. However, a similar representation of antibodies to measles virus components was also seen in sera from patients with active chronic hepatitis. The significance of this finding for the interpretation of a weak antibody response to M antigen in the presence of a pronounced antibody response to other components is discussed.

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Immunofluorescence has been used to study visceral organs from a case of subacute sclerosing panencephalitis. Immune complexes were shown as granular deposits of IgG, complement, and measles antigens in renal glomeruli. Measles antigens were detected in the spleen, liver, and lymph nodes from many parts of the body.

Immune-complex formation may be important in the aetiology of this disease and perhaps in causing some of its tissue damage. The rarity of subacute sclerosing panencephalitis may be due to an unusual pattern of immunological reactivity required in a patient before a measles infection can produce a subacute encephalitis.

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Subacute sclerosing panencephalitis (SSPE) is a lethal disease induced by the persistence of measles virus in the human brain. In many SSPE cases, the viral matrix (M) protein cannot be detected; in others, M proteins of the expected size are found and sequence analysis of M cDNAs has confirmed that the reading frames are intact, showing only several missense mutations. To determine whether these alterations result in nonfunctional proteins, we have replaced the M gene of an infectious full-length genomic cDNA (from vaccine strain Edmonston) with different M genes derived from four patients with SSPE. One of the SSPE M genes tested proved to be functionally competent, giving rise to a virus yielding titers similar to those of viruses containing the M gene from control lytic strains. The other three SSPE M genes were not functionally competent in the same test. In all three cases, the inactivating changes resided in the carboxyl-terminal half of the M protein, as shown by the exchange of either of the two genes halves. In summary, mutational M gene alterations, which either prevent synthesis of M protein altogether or only allow synthesis of nonfunctional M protein, have been detected by us and by others in 9 of 10 SSPE cases. The one functional M gene appears to be an exception to the rule, indicating that M gene alteration might not be an absolute requirement for disease development.

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A 20-year-old girl developed a subacute neurological illness characterized by seizures and epilepsia partialis continua, which resulted in her death within 10 weeks of her first symptom. Although she had a history of unusual reactions to viral infections, there was no evidence of any underlying disorder resulting in immunosuppression. Histopathology demonstrated the presence of dense infection with measles virus. The unusual clinical features of this cases suggest that measles virus may be responsible for a wide spectrum of neurological disease ranging from measles inclusion body encephalitis on the one hand to subacute sclerosing panencephalitis on the other.

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Neurologic diseases are important complications of measles. The role of virus infection of the central nervous system as well as the route of virus entry has been unclear. Five autopsied cases of individuals who died with severe acute measles 3-10 d after the onset of the rash were studied for evidence of viral involvement of the central nervous system. In all cases, in situ hybridization and RT-PCR in situ hybridization techniques showed endothelial cell infection. Immunoperoxidase staining with an anti-ferritin antibody revealed a reactive microgliosis. These data suggest that endothelial cells in the brain are frequently infected during acute fatal measles. This site of infection may provide a portal of entry for virus in individuals who subsequently develop subacute sclerosing panencephalitis or measles inclusion body encephalitis and a target for immunologic reactions in post-measles encephalomyelitis.

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A child with acute lymphoblastic leukaemia, being treated in the UKALL II Trial, had while in remission an attack of measles and made a normal recovery. Four months later she developed an acute encephalopathy and died within two weeks. The brain showed mild inflammatory features and widespread inclusion bodies in neurones and glial cells. Immunofluorescence proved an infection with measles virus. Similar cases have been called SSPE; reasons are given for preferring the term "measles inclusion-body encephalitis".

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We studied a variety of patients with measles virus infection by using avidity testing for measles virus-specific immunoglobulin G (IgG) in serum and cerebrospinal fluid samples. For the avidity testing, an Enzygnost measles IgG enzyme-linked immunosorbent assay kit was used with an 8 M urea denaturing method. With this method, low-avidity IgG (acute primary infection, avidity of < 30% within 15 days of the onset of rash) and high-avidity IgG (subacute sclerosing panencephalitis, avidity of > 75%) could be clearly distinguished by using serum samples. One patient, who developed a typical course of measles despite a previous vaccination, showed a positive IgM response with an initial low titer of measles virus-specific IgG of low avidity, but a later sample revealed a high titer of IgG of intermediate (40%) avidity, suggesting previous immunological priming. Two patients with breakthrough infection (secondary vaccine failure), both having central nervous system involvement, showed a positive IgM response with initial high titers of serum IgG of high avidity. In addition, one of the patients had a detectable level of measles-specific IgG in cerebrospinal fluid. In this patient, the avidity of both serum and cerebrospinal fluid IgG decreased during the short follow-up period. This phenomenon has never before been reported. In subacute sclerosing panencephalitis patients, the avidity of cerebrospinal fluid IgG was consistently lower than that of serum IgG. The difference in avidity between cerebrospinal fluid and serum IgG may be used as a direct indicator of intrathecal production of IgG. In conclusion, the avidity testing is simple to perform, reliable, and highly informative in the analysis of measles virus infection.

Stored serum specimens, from four regions of Thailand, of healthy children attending well baby clinics and of healthy people with acute illnesses visiting outpatient clinics were randomly sampled and tested for IgG antibody to measles, mumps, and rubella (MMR). The immunity patterns of rubella and mumps fitted well with the history of rubella and MMR vaccination, seroprotective rates being over 85% among those aged over seven years. A high proportion of younger children acquired the infection before the age of vaccination. MMR vaccination should preferably be given to children at an earlier age. For measles, 73% seroprotective rates among children, aged 8-14 years, who should have received two doses of measles/MMR vaccine, were lower than expected. This finding was consistent with the age-group reported in outbreaks of measles in Thailand. The apparent ineffectiveness (in relation to measles) of MMR immunization of 1st grade students warrants further studies.

Measles virus (MV) is the causative agent for acute measles and subacute sclerosing panencephalitis (SSPE). Although numerous mutations have been found in the MV genome of SSPE strains, the mutations responsible for the neurovirulence have not been determined. We previously reported that the SSPE Osaka-2 strain but not the wild-type strains of MV induced acute encephalopathy when they were inoculated intracerebrally into 3-week-old hamsters. The recombinant MV system was adapted for the current study to identify the gene(s) responsible for neurovirulence in our hamster model. Recombinant viruses that contained envelope-associated genes from the Osaka-2 strain were generated on the IC323 wild-type MV background. The recombinant virus containing the M gene alone did not induce neurological disease, whereas the H gene partially contributed to neurovirulence. In sharp contrast, the recombinant virus containing the F gene alone induced lethal encephalopathy. This phenotype was related to the ability of the F protein to induce syncytium formation in Vero cells. Further study indicated that a single T461I substitution in the F protein was sufficient to transform the nonneuropathogenic wild-type MV into a lethal virus for hamsters.

Subacute sclerosing panencephalitis (SSPE) is a rare, slowly progressive neurological disorder caused by the persistent infection with measles virus (MV). Despite much research into SSPE, its pathology remains obscure. We examined autopsy tissues of eight SSPE patients by real time quantitative PCR, immunohistochemistry and immunoblotting to determine viral load. MV N, M and H gene RNA could be detected in the central nervous system (CNS) of all patients and in two non-CNS tissues of one patient. The viral burden between patients differed up to four-fold by quantitative PCR and corresponded with detection of MV protein. The level of both viral RNA and antigen in the brain may correlate with disease progression.

Interaction between the Edmonston or Nagahata strain of acute measles virus (MV) and the defective Biken strain of MV isolated from a patient with subacute sclerosing panencephalitis (SSPE) was examined by a cell fusion protocol. Biken-CV-1 cells nonproductively infected with Biken strain SSPE virus were fused with neomycin-resistant CV-1 cells. All the fused cells selected with the neomycin analog G418 expressed Biken viral proteins, as determined by an immunofluorescence assay. This procedure enabled the transfer of Biken viral genomes into cells previously infected with MV. In the fused cells coinfected by Biken strain SSPE virus and Edmonston or Nagahata strain MV, early MV gene expression was suppressed, as determined by immunoprecipitation with strain-specific antibodies. Maturation of Edmonston strain MV was also suppressed. When the coinfected fused cells were selected with G418, Biken viral proteins remained at a constant level for up to 7 weeks. Wild-type MV proteins gradually decreased to a barely detectable level after 4 weeks and became undetectable after 7 weeks. Immunofluorescence studies showed a steady decline in cells expressing wild-type MV proteins in the coinfected cultures. These results suggest that Biken strain SSPE virus dominantly interferes with the replication of wild-type MV. The possible mechanisms of dominant interference and the implication for evolution of a persistent MV infection are discussed.

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Infection by measles virus (MV) is a major cause of human morbidity and mortality worldwide. In 2001, the WHO, UNICEF and their partners launched the Measles Initiative, the goals of which are to interrupt the transmission of MV in large geographic areas by increasing vaccination coverage and to assess the feasibility of eradicating MV worldwide. An estimated 74% reduction in mortality resulting from measles was achieved between 2000 and 2007, equivalent to a reduction of approximately 200,000 deaths annually. Despite this progress in the control of measles, the highest number of measles cases in more than a decade was observed in 2008 in several European countries and the US, and the virus was again declared endemic in the UK. In the light of this resurgence in the UK and the limitations associated with the current live-attenuated vaccine, this review discusses the means by which safe and effective measles antivirals could augment vaccination and strengthen global efforts to control measles. Important aspects of treatment are the potential to prevent infection effectively after exposure to MV, the improvement of case management, the amelioration of complications that frequently follow MV infection and the influence of antivirals on a potential strategy for global measles eradication.

We have developed an in vitro nucleocapsid-binding assay for studying the function of the matrix (M) protein of measles virus (MV) (A. Hirano, A. H. Wang, A. F. Gombart, and T. C. Wong, Proc. Natl. Acad. Sci. USA, 89:8745-8749, 1992). In this communication we show that the M proteins of three MV strains that cause acute infection (Nagahata, Edmonston, and YN) bind efficiently to the viral nucleocapsids whereas the M proteins of four MV strains isolated from patients with subacute sclerosing panencephalitis (SSPE) (Biken, IP-3, Niigata, and Yamagata) fail to bind to the viral nucleocapsids. MV Biken (an SSPE-related virus) produces variant M sequences which encode two antigenically distinct forms of M protein. A serine-versus-leucine difference is responsible for the antigenic variation. MV IP-3 (an SSPE-related virus) also produces variant M sequences, some of which have been postulated to encode a functional M protein responsible for the production of an infectious revertant virus. However, the variant M proteins of Biken and IP-3 strains show no nucleocapsid-binding activity. These results demonstrate that the nucleocapsid-binding function is conserved in the M proteins of MV strains that cause acute infection and that the M proteins of MV strains that cause SSPE exhibit a common defect in this function. Analysis of chimeric M proteins indicates that mutations in the amino-terminal, carboxy-proximal, or carboxy-terminal region of the M protein all abrogate nucleocapsid binding, suggesting that the M protein conformation is important for interaction with the viral nucleocapsid.

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In patients with subacute sclerosing panencephalitis (SSPE), which is associated with persistent measles virus (MV) infection in the brain, little infectious virus can be recovered despite the presence of viral RNA and protein. Based on studies of brain tissue from SSPE patients and our work with MV-infected NSE-CD46+ mice, which express the measles receptor CD46 on neurons, several lines of evidence suggest that the mechanism of viral spread in the central nervous system differs from that in nonneuronal cells. To examine this alternate mechanism of viral spread, as well as the basis for the loss of normal transmission mechanisms, infection and spread of MV Edmonston was evaluated in primary CD46+ neurons from transgenic mice and differentiated human NT2 neurons. As expected, unlike that between fibroblasts, viral spread between neurons occurred in the absence of syncytium formation and with minimal extracellular virus. Electron microscopy analysis showed that viral budding did not occur from the neuronal surface, although nucleocapsids were present in the cytoplasm and aligned at the cell membrane. We observed many examples of nucleocapsids present in the neuronal processes and aligned at presynaptic neuronal membranes. Cocultures of CD46+ and CD46− neurons showed that cell contact but not CD46 expression is required for MV spread between neurons. Collectively, these results suggest that the neuronal environment prevents the normal mechanisms of MV spread between neurons at the level of viral assembly but allows an alternate, CD46-independent mechanism of viral transmission, possibly through the synapse.

The persistence of measles virus in selected areas of the brains of four patients with subacute sclerosing panencephalitis (SSPE) was characterized by immunohistological and biochemical techniques. The five measles virus structural proteins were never simultaneously detectable in any of the brain sections. Nucleocapsid proteins and phosphoproteins were found in every diseased brain area, whereas hemagglutinin protein was detected in two cases, fusion protein was detected in three cases, and matrix protein was detected in only one case. Also, it could be shown that the amounts of measles virus RNA in the brains differed from patient to patient and in the different regions investigated. In all patients, plus-strand RNAs specific for these five viral genes could be detected. However, the amounts of fusion and hemagglutinin mRNAs were low compared with the amounts in lytically infected cells. The presence of particular measles virus RNAs in SSPE-infected brains did not always correlate with mRNA activity. In in vitro translations, the matrix protein was produced in only one case, and the hemagglutinin protein was produced in none. These results indicate that measles virus persistence in SSPE is correlated with different defects of several genes which probably prevent assembly of viral particles in SSPE-infected brain tissue.

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The authors report a fatal case of disseminated tuberculosis in a 14-yr-old girl, which developed immediately after a measles-rubella (MR) vaccination. Despite a markedly accelerated clinical course which led to death within two weeks, the authors could not identify any possible cause of the tuberculosis aggravation in this case, with the exception of the MR vaccination. The possible role that MR vaccination had on the clinical course of tuberculosis in this case is discussed.

The anti-measles virus (MV) antibody titers in the sera of vaccinees and naturally infected individuals of different age groups were measured to help assess the efficacy of the current MV vaccination in Japan. Neutralizing (NT) antibody titers induced by vaccination were 23.2 times lower than those induced by natural infection and declined significantly by age 20. The once-decreased NT antibody titers of the vaccinees increased 23.6 times during their twenties to titers comparable to those of naturally infected individuals of the same age, implying the possible occurrence of natural infection in vaccinees with decreased anti-MV immunity. Although the current field strains in Japan, types D3 and D5, were reported to differ antigenically from each other and from vaccine strains (type A) to some extent, as demonstrated by different reactivities to monoclonal antibodies, the sera of vaccinees neutralized the two types of field strains and the vaccine strain with the same efficiency. This result suggests that the current vaccine strain would be suitable to elicit protection against types D3 and D5, as long as viral antigenicity is concerned. However, when compared at given hemagglutination inhibition titers, NT antibody titers of vaccinees were 21.1 to 23.2 times lower than those of naturally infected individuals, suggesting a qualitative difference(s) of anti-MV antibodies between the two groups. It should be emphasized that protective immunity induced by the one-dose vaccination currently implemented in Japan may not be strong enough to ensure lifelong immunity. A two-dose vaccination program with higher vaccination coverage needs to be considered in order to effectively control measles in Japan.

Background

Strong regional heterogeneity and generally sub-optimal rates of measles vaccination in Italy have, to date, hampered attainment of WHO targets for measles elimination, and have generated the need for the new Italian National Measles Elimination Plan. Crucial to success of the plan is the identification of intervention priorities based upon a clear picture of the regional epidemiology of measles derived from the use of data to estimate basic parameters. Previous estimates of measles force of infection for Italy have appeared anomalously low. It has been argued elsewhere that this results from Italian selective under-reporting by age of cases and that the true measles force of infection in Italy is probably similar to that of other European countries. A deeper examination of the evidence for this conjecture is undertaken in the present paper.

Methods

Using monthly regional case notifications data from 1949 to the start of vaccination in 1976 and notifications by age from 1971–76, summary equilibrium parameters (force of infection (FOI), basic reproductive ratio (R0) and critical vaccination coverage (pc)) are calculated for each region and for each of 5 plausible contact patterns. An analysis of the spectra of incidence profiles is also carried out. Finally a transmission dynamics model is employed to explore the correspondence between projections using different estimates of force of infection and data on seroprevalence in Italy.

Results

FOI estimates are lower than comparable European FOIs and there is substantial regional heterogeneity in basic reproductive ratios; certain patterns of contact matrices are demonstrated to be unfeasible. Most regions show evidence of 3-year epidemic cycles or longer, and compared with England & Wales there appears to be little synchronisation between regions. Modelling results suggest that the lower FOI estimated from corrected aggregate national data matches serological data more closely than that estimated from typical European data.

Conclusion

Results suggest forces of infection in Italy, though everywhere remaining below the typical European level, are historically higher in the South where currently vaccination coverage is lowest. There appears to be little evidence to support the suggestion that a higher true force of infection is masked by age bias in reporting.

Background

Mass vaccination against measles has successfully lowered the incidence of the disease and has changed the epidemic pattern from a roughly biennial cycle to an irregular sequence of outbreaks. A possible explanation for this sequence of outbreaks is that the vaccinated population is protected by solid herd immunity. If so, we would expect to see the fraction of susceptible individuals remaining below an epidemic threshold. An alternative explanation is the occurrence of occasional localised lapses in herd immunity that allow for major outbreaks in areas with a low vaccine coverage. In that case, we would expect the fraction of susceptible individuals to exceed an epidemic threshold before outbreaks occur. These two explanations for the irregular sequence of measles outbreaks can be tested against observations of both the fraction of susceptible individuals and infection attack rates.

Methods and Findings

We have estimated both the fraction of susceptible individuals at the start of each epidemic year and the infection attack rates for each epidemic year in the Netherlands over a 28-y period. During this period the vaccine coverage averaged 93%, and there was no sustained measles transmission. Several measles outbreaks occurred in communities with low vaccine coverage, and these ended without intervention. We show that there is a clear threshold value for the fraction of susceptible individuals, below which only minor outbreaks occurred, and above which both minor and major outbreaks occurred. A precise, quantitative relationship exists between the fraction of susceptible individuals in excess of this threshold and the infection attack rate during the major outbreaks.

Conclusion

In populations with a high but heterogeneous vaccine coverage, measles transmission can be interrupted without establishing solid herd immunity. When infection is reintroduced, a major outbreak can occur in the communities with low vaccine coverage. During such a major outbreak, each additional susceptible individual in excess of the threshold is associated with almost two additional infections. This quantitative relationship offers potential for anticipating both the likelihood and size of future major outbreaks when measles transmission has been interrupted.

High vaccine coverage does not necessarily protect against major outbreaks of measles in unvaccinated individuals in areas where vaccine coverage is low.