The notified AEFI seem to be associated more with the type of vaccine than with the child's age. The first vaccine (HB) is given on the first postnatal day, and this showed the second highest rate of AEFI, but the vaccine that produced the highest rate of AEFI (DTP/DTwP/Hib) is usually administered at two months of age. It is important to emphasize that in Brazil the rate of reported AEFI is positively associated with the human development index of the state [10
], suggesting that underreporting occurs. Indeed, Gomes Monteiro et al. [10
] showed the lowest reported AEFI of DTP (from 2002 to 2005) to be in the Northern region of Brazil which has the lowest development indices in the country. Therefore, it is likely that the notified AEFI are underestimated for the state of Rondonia, a representative Northern state.
Fever, HHE, and seizures were the most common systemic AEFI reported in this study. Although in the case of DTwP/Hib (along with convulsions) recovery without after effects was achieved in 98.4% of the cases [11
], practical counseling does exist, which can help improve management of cases. Fever, irritability, and feeding disturbances (even anorexia) are transient adverse events that are widely observed, especially in children aged less than seven months, and these events may impact breastfeeding rates and nutrition; in such cases it is important to remind parents that breastfeeding protects against decreased energy intakes, decreasing pain and alleviating discomfort and stress of vaccination in very young children [12
AEFI can vary as a function of the vaccine (formulation and manufacturers), vaccinee (age of children), and country (vaccination scheme adopted, AEFI reporting systems, and adverse event compensation policies); hence the difficulty in comparing outcomes of AEFI within and between countries. Because of such difficulties, this is a subject that has not been well studied [3
], and we can only make inferences from indirect sources. Evans [13
] compiled information on compensations available for a few countries and showed that compensated claims (which refer to severe AEFI) by vaccine type differ greatly. It seems that DTP was, at the time of the study, the vaccine showing the highest claims [13
] and still is the vaccine with the highest occurrence of AEFI in young children.
For few individuals AEFI are severe, acute, with outcome clearly perceived as harmful. When a severe adverse event happens, an individual bears a significant burden for the greater good or “herd immunity” [14
]. Concerns with the long-lasting after effects of disabling illness caused by or associated with the use of vaccines have led a number of countries to create effective systems of surveillance for AEFI and respective compensation programs [14
During the last decade, however, we have seen an increased awareness related to AEFI that has extended beyond specific vaccine antigens to include low doses of excipients (preservative-Thimerosal and adjuvant-aluminum). Both ethylmercury (a breakdown product of Thimerosal) and aluminum are known neurotoxicants per se
and, in the case of Hg, with a long history of known toxicity that also includes its organic form—ethylmercury [15
]. These excipients (with neuronal effects) are used at low doses as part of some vaccine formulations (Thimerosal-containing vaccines [TCV], and aluminum-adjuvanted vaccines) and are considered safe. Indeed none of the rare neurologic adverse events (encephalopathy, Guillain-Barre syndrome, meningo-encephalitis, polyneuropathy, peripheral neuritis, per se
or in combination) associated with vaccine-antigens [16
] can be attributed to low doses of either mercury or aluminum. However, the untested concept of low-dose safety of these excipients originated in the 1930s in the wake of vaccine development. New experimental research designed to model low-dose exposure relevant to vaccines has established proof-of-concept that Thimerosal-Hg has the potential to produce nonclinical effects in the central nervous system [17
] not contemplated by AEFI.
Since late 1990s industrialized countries of Europe and North America have restricted the use of Thimerosal as a preservative in vaccines intended for infants and young children. Although experimental studies can demonstrate toxic effects of low doses of Thimerosal we cannot predict neurological disorders for vaccine-Thimerosal. The few epidemiological studies taken together can at best be interpreted as inconclusive; they cannot show a clear association of ethylmercury with mental disability [18
]. However, several studies point to transient delays in neurodevelopment as measurable by neurobehavioral tests [20
] as well as decreased pain associated with Thimerosal-free vaccine [24
]. Additionally, regarding immunologic effects, epidemiological studies have suggested an association of Thimerosal and patch-test sensitivity; countries that eliminated Thimerosal from vaccines such as Austria, Denmark, Poland, and the USA [26
] observed a decrease in Thimerosal patch-test reactions. Moreover, both experimental [27
] and clinical [28
] studies have addressed autoimmune (autoinflammatory) syndrome induced by adjuvants (ASIA) in association with some adjuvanted vaccines. Although this emerging information is not part of the AEFI recording system the collected data are nevertheless shaping perceptions among health workers and stakeholders.
The importance of the study is that it the first attempt to address the AEFI after its implementation. The main limitation is that this system in the state of Rondonia, one with a low score of developmental index, has no means to assess reporting bias (i.e., full medical histories of patients) or to estimate nonreported adverse events.