Primary vaccine failure—24% in the present study—is evidently common in healthy young children after a single dose of mono-valent varicella vaccine. The 76% VZV seroconversion rate, determined with the FAMA assay, is significantly lower than rates obtained with the gpELISA assay, which is commonly used because it is easy to conduct and can be automated [7
]. Most reported gpELISA seroconversion rates have been >95% when measured 6 weeks after 1 dose of monovalent varicella vaccine and using a value of ≥5 gpELISA units/mL to indicate a positive response [9
]. Only one study found a lower rate, of 86% [4
]. These high gpELISA-determined seroconversion rates do not correspond well with the demonstrated effectiveness of varicella vaccine, which is 80%–85% [25
]. The current observations made using the FAMA assay to determine immunity are consistent with epidemiologic experience and indicate that most breakthrough cases of varicella after a single dose of vaccine can be attributed to primary vaccine failure.
The FAMA assay was initially validated as an indicator of immunity to varicella in 1974 [11
] and was the first serologic assay that made it possible to determine susceptibility to varicella. As such, the FAMA assay is considered the gold standard to which the sensitivity and specificity of other assays of varicella immunity are compared [25
]. A seroconversion rate of 100% has been found by the FAMA assay to occur in >1000 children and adults after developing clinical varicella [11
]. Because varicella is highly contagious [33
], protection after household exposure was used to evaluate immunity by means of the FAMA assay. A positive FAMA titer (≥1:4) predicts protection from infection after household exposure to VZV with rare exceptions
]. The attack rate of chickenpox in FAMA-positive individuals after household exposure in an aggregate of multiple studies is 2% (3/126), whereas that in FAMA-negative individuals is 59% (37/63) [11
]. Among the FAMA-negative population, the attack rate in unvaccinated individuals with no history of varicella is 77% (20/26) [11
]. The attack rate in vaccinated adults with a titer of <1:4 is 46% (17/37) [12
]. FAMA-negative vaccinees thus have a significantly lower attack rate after household exposure than do individuals who are naturally susceptible (46% vs. 77%; P
< .01). Cell-mediated immunity induced by vaccination may thus provide partial protection to some vaccinees and may mitigate infection, even though individuals lack antibodies detectable by the FAMA assay at the time of exposure. This phenomenon might also explain why many, but not all, vaccinees who develop break-through varicella have a mild illness [39
]. The FAMA data presented here nevertheless indicate that the incidence of primary vaccine failure after a single dose of varicella vaccine is substantial.
In contrast to the FAMA assay, the sensitivity and specificity of the gpELISA method has not been validated in the context of protection after household exposure to varicella. There is, moreover, evidence that gpELISA yields false-positive results when used to determine varicella immunity. When the gpELISA method was first developed, 14% of young children with no history of chickenpox or immunization were found to have positive titers [7
]. These children were postulated to have acquired antibodies to varicella as a result of retained maternal antibodies and episodes of subclinical varicella. Neither explanation is likely because maternal antibodies are lost before 9 months of age and subclinical varicella is rare [33
]. It is more plausible that the end point used to indicate immunity was too low, causing false-positive results to occur in gpELISAs. Inclusion of false-positive results would lead to overestimations of the rate of VZV sero-conversion when assessed by the gpELISA method. Such an overestimate would account for the difference in seroconversion rates estimated by the FAMA assay and gpELISA.
We have been unable to compare directly gpELISA and FAMA titers in vaccinated children because we do not have access to the reagents to perform the gpELISA. A relevant comparison, however, performed some years ago, was done in adults. In a series of serum samples from 17 adults who developed varicella, all were FAMA-negative before becoming ill [12
]. When the same serum samples were tested by gpELISA in another laboratory, 12% were found to exhibit titers compatible with immunity to varicella (≥5 gpELISA units/mL) (A.A.G. and S.P.S., unpublished data). In a published series of serum samples from vaccinees tested by other investigators, only 60% of 41 postvaccine titers judged to be moderately positive by gpELISA were also found to be positive by FAMA assay. Of serum samples from 16 patients with high postimmunization gpELISA titers, only 94% were FAMA positive [7
]. These results are consistent with the idea that, when judged by the FAMA comparator, the gpELISA over-estimates immunity to varicella. Consideration of the value of the gpELISA assay has been made more difficult because the threshold used for considering the test to be read as positive has varied from 0.625 to ≥5 gpELISA units/mL in different publications [4
]. A published correlation between neutralization of VZV and gpELISA used 0.3 units/mL as a negative value and 0.625 units/mL as a positive value [8
]. The positive value of 0.625 units/mL, however, has now been abandoned and does not predict immunity. The earlier validation of the gpELISA using a VZV neutralization assay thus cannot be relied on; neutralization tests for VZV are difficult to perform [25
Numerous outbreaks of varicella have occurred in vaccinated populations in the United States since the vaccine was licensed, indicating that protection from varicella is not as effective as was originally expected. Since 1997, 18 outbreaks have been reported [14
]. Many other outbreaks of varicella have undoubtedly not been reported. Vaccine effectiveness in reported outbreaks ranged from 44% to 100%, with an average of 79% [25
]. In only 1 of 18 outbreaks was effectiveness >90%, despite seroconversion rates being as high as 96% by gpELISA. Protection in the setting of an exposure is thus significantly lower than the gpELISA seroconversion data would lead one to expect.
The most plausible explanations for the observed degree of vaccine failure after 1 dose of vaccine are that immunized children either do not develop humoral immunity to VZV at all or that there is an initial immune “burst” of immunity that is enough to generate a positive gpELISA result but is inadequate to generate a sustained memory T cell response. This concept is consistent with data from a case-control study in which vaccine effectiveness was 97% though the first year after vaccination but fell to 86% in the second year. Although there was some small decline of effectiveness in subsequent years, these declines were not statistically significant [26
The present study suggests that VZV antibody titers may decrease during the first year after vaccination. There may be a rapid initial fall in VZV GMTs in a subset of patients. Although there is some overlap in ages of children in different locales, children tested 3 months after vaccination had higher median FAMA GMTs (1:12.4) than did those tested 6 months after vaccination (1:4.9). Further studies should be undertaken to see whether these findings can be confirmed. Whether or not a decline continues beyond 6 months, moreover, remains to be demonstrated. Secondary vaccine failure, defined as a demonstrable immune response to vaccination that gradually wanes over a long period of time, cannot explain the high percentage of children who lack FAMA-detectable VZV antibodies in the early weeks and months after immunization. It is possible that secondary vaccine failure occurs, but it has not been consistently demonstrated [26
]. Our present data, however, do not directly address the issue of whether or not secondary vaccine failure occurs. However, because primary vaccine failure occurs in 24% of children after a single dose of vaccine, if secondary vaccine failure does occur, it is likely to be less significant than primary vaccine failure given the recognized vaccine effectiveness of 85%. It should be noted that other published data showing a lower rate of primary immune failure by use of the FAMA assay did not use our exact procedure for the FAMA test. In the assay used by Johnson et al. [43
], VZV-infected cells used as the antigen were dried on slides. Our assay uses live, unfixed cells, presumably preserving the protein conformation of VZV glycoproteins on the cell surface [11
It is conceivable that improper storage of the vaccine explains the relatively high rate of primary vaccine failure found in the present study. Low FAMA seroconversion rates, however, were detected in 3 different practice settings. It seems unlikely that all 3 would have allowed the vaccine to deteriorate. Still, the overall number of patients investigated is not large, and the number of sites, although multiple, may not be totally reflective of the universe of sites at which vaccination is done. A larger study of vaccines in widely distributed sites should thus be conducted to evaluate the rate of primary vaccine failure in the entire United States.
Primary vaccine failure in just 10% of vaccinees after a single dose could result in progressive accumulation of susceptible individuals over time and lead to an increased incidence of varicella in young adults. Such an increment is potentially dangerous. Approximately 4 million infants are vaccinated annually in the United States. A primary vaccine failure rate of 10% would thus lead to 400,000 vaccinated but susceptible infants every year. Within 5 years, there would be 2 million vaccinated but susceptible individuals. The present findings therefore strongly support the use of a second dose of vaccine for all children without a history of disease. We thus support the recent recommendation made by the Advisory Committee on Immunization Practices of the Centers for Disease Control and Prevention that a second dose of varicella vaccine be given to all children routinely. The unacceptably high rate of primary vaccine failure suggests that the interval between the first and second doses of vaccine should be a matter of months rather than years. Correction of the problem of primary vaccine failure is thus urgent; if done it will probably prevent not only the current phenomenon of isolated outbreaks of breakthrough disease but a subsequent epidemic of serious varicella in vaccinated but unprotected adults.