However, data provided by Winner and Evans (1990) and Hankey (1987) suggest the background incidence rate for children varies by age interval. Based on the latter data, the risk difference in young children could be significantly greater than that based on the background incidence described by Uhari et al. and could approach that in adults. Relatively few adults in the United States receive OPV; thus, the proportion of GBS cases in adults attributable to OPV would be small.

Conclusion

The evidence favors acceptance of a causal relation between OPV and GBS. The relative risk on the basis of studies done in Finland is on the order of 3.5 for adults, and the risk difference is approximately 2.5 per 100,000 people. Estimates of background incidence rates for GBS in children vary. It is not clear what the relative risk and risk difference are for children in the United States (see Chapter 3).

The evidence is inadequate to accept or reject a causal relation between IPV and GBS.

Risk-Modifying Factors

GBS, as a separate discrete attack, recurs in a small percentage of those previously afflicted, perhaps 2 to 3 percent, and some individuals have been known to have three or four separate episodes. Other than the patient described by Pollard and Selby (1978), who experienced three attacks, each within 10-21 days of receipt of tetanus toxoid, cases of recurrence after vaccination are not documented. Nevertheless, if GBS occurs within 5 days to 6 weeks of a vaccination, subsequent vaccinations with either OPV or different immunogens could be associated with a greater risk of GBS than if the person had never had GBS. A previous history of GBS unrelated to vaccination as an antecedent event is even more uncertain as a risk factor.

ANAPHYLAXIS AND THROMBOCYTOPENIA

Clinical Description

Anaphylaxis is a sudden, potentially life-threatening systemic condition mediated by highly reactive molecules from mast cells and basophils. The clinical manifestations of anaphylaxis include pallor and then diffuse erythema, urticaria and itching, subcutaneous edema, edema and spasm of the larynx, wheezing, tachycardia, hypotension, and hypovolemic shock, usually occurring within minutes of intramuscular or subcutaneous exposure to antigen. See Chapter 4 for a more detailed discussion of anaphylaxis.



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