Adverse Events after Vaccination

Adverse Events after Vaccination

Frequency of Adverse Events after Vaccination with Different Vaccinia Strains by Mirjam Kretzschmar et al

Below is the Editor's Summary. Click here for the full article.

Background

For thousands of years, smallpox was one of the world's most-feared diseases. This contagious disease, caused by the variola virus, historically killed about 30 percent of the people it infected. Over the centuries, it probably killed more people than all other infectious diseases combined, but it was also the first disease to be prevented by vaccination. In 1796, the English physician Edward Jenner rubbed pus from the spots of a milkmaid with cowpox into scratches on a young boy's arm; according to folklore, people who caught cowpox, a related but mild disease of cows, were protected against smallpox. Six weeks later, after a mild bout of cowpox, when the boy was challenged with pus from a smallpox patient, he did not develop smallpox. This vaccination procedure was later refined so that people were inoculated with pure preparations of live vaccinia virus, which is closely related to the smallpox and cowpox viruses, and by 1979 a global vaccination campaign had totally eradicated the disease.

Why Was This Study Done?

Smallpox vaccination has some adverse effects. In particular, vaccinia virus occasionally infects the brain. This so-called post-vaccination encephalitis can cause permanent brain damage and, it has been estimated, kills one vaccinee in every million. Consequently, as smallpox became rarer, the dangers of vaccination began to outweigh its benefits. Routine smallpox vaccination stopped in the US in 1972, and in 1980 the World Health Organization recommended that all countries stop vaccination. Now, however, there are fears that smallpox may be used for bioterrorism. If this did happen, exposed individuals and their contacts, possibly even whole populations, would have to be vaccinated as quickly as possible (very few people now have strong immunity to smallpox). Many countries have stockpiles of smallpox vaccines for this eventuality, but these contain different vaccinia virus strains. In this study, the researchers examined historical data to discover whether these strains differ in their potential to cause encephalitis and death. This information should help public-health officials plan their vaccination strategies in response to a bioterrorism attack with smallpox.

What Did the Researchers Do and Find?

The researchers collected data from published studies on smallpox vaccination and adverse events from several countries from the late 1950s onwards. They then used these data to extrapolate how often the different vaccinia strains might cause encephalitis and death if they were used today in vaccination programs. They estimate that vaccinating with the New York City Board of Health (NYCBH) strain, which is stockpiled in the US, might cause 2.9 cases of post-vaccination encephalitis and 1.4 deaths per million vaccinated individuals. In contrast, the Lister strain, which is stockpiled in many European countries, might cause 26.2 cases of post-vaccination encephalitis and 2.5 deaths per million vaccinees. For both strains, vaccination of children younger than 1 year old would cause the highest death rate, and individuals being re-vaccinated would be less likely to die than those being vaccinated for the first time. Finally, the researchers use their figures to estimate that about ten people would die if mass vaccination with the NYCBH strain were used in the Netherlands (population 16 million), whereas 55 people would die if the Lister strain were used.

What Do These Findings Mean?

The data used in this study are of variable quality, so the figures calculated by the researchers are only estimates. For instance, given the scatter of the original data, mass vaccination in the Netherlands with the Lister strain might cause anywhere between seven and nearly 200 deaths. However, the study clearly suggests that more serious adverse events would occur after vaccination with the Lister strain than after vaccination with the NYCBH strain. It also indicates that even in the US, where the NYCBH vaccine strain is stockpiled, previous analyses of the effects of vaccination in response to a bioterrorist attack have probably underestimated how many people might die from post-vaccination encephalitis. Public-health decision makers should incorporate these new estimates into their planning for a smallpox outbreak. These increased estimates of adverse events after vaccination might, for example, make mass vaccination with the Lister strain of vaccinia virus less acceptable. Instead, public-health officials might decide to rely on vaccination of only the people directly exposed to released smallpox virus and their close contacts (ring vaccination) to contain a smallpox outbreak.

Additional Information

Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0030272.

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