Information for health-care workers - managing adverse events
Reasons for misleading information about rates
Millions of doses of a vaccine may be administered during a mass vaccination campaign or supplementary immunization activity (SIA). Even if adverse events occur at the same rate as for the rest of the year, it could be anticipated that this will result in a more concentrated occurrence of AEFIs than usual, even if the rate stays the same. For instance, if the normal rate of sore arms following administration of measles vaccine is twelve sore arms per 100,000 doses of measles vaccine and if a country gives on average 100,000 doses of measles vaccine in one year, then we would expect those twelve sore arms to be reported over the year, approximately one per month. If a campaign is carried out when one million doses of measles vaccine are given in a week, we would expect 120 cases of sore arms to be reported in the week if the rate stays the same as usual.
To the outside observer this might appear as an alarming outbreak of sore arms associated with the campaign. In fact, the rate is unchanged. This concept is important for programme managers to get across to staff, politicians and parents.
If we apply this idea to serious events such as paralysis following administration of oral polio vaccine (OPV) during or soon after National Immunization Days (NIDs), there is the potential for the situation to create grave concern in the community. The rate of vaccine-related paralysis following administration of OPV is of the order of one case per two-three million doses administered. During NIDs in China and India in 1996, around 160 million doses of OPV were administered in one week. It would not have been unusual to have had reported up to 50-80 cases of paralysis following the NIDs. In actual fact, very few cases were reported (some may have occurred but not been reported). Programme managers were aware of the expected rate and were able to monitor reports of AEFIs accordingly. It is important to balance the occurrence of the few cases of vaccine-associated adverse events with the much larger number of cases of naturally-occurring polio which would have occurred if the NID had not taken place.
Following an NID in a European country early in 1996, cases of paralysis were reported after they had received OPV. However, on laboratory analysis, the wild virus was found to be responsible for the cases. An outbreak had occurred at the same time as the NID and was confusing the picture. When the public was reassured by the Minister of Health and the President that a reliable external laboratory had performed the tests, a second NID was undertaken and successfully prevented more cases of polio from occuring.