Preliminary results from two 2009 influenza A(H1N1) vaccine trials in Britain and Australia have shown that the vaccines tested on adults are both safe and effective. Doctors and health workers in some countries had expressed concern over the safety of a fast-tracked vaccine and had reservations about being vaccinated. Although the two vaccines had different dosages, designs, and composition, they were well tolerated by all the volunteers. The most common adverse reaction was tenderness at the site of vaccination. Preliminary analyses of data from two vaccine trials involving 2,800 adult volunteers in the United States have also shown that vaccines are well tolerated. The European Medicines Agency and the U.S. regulatory authority note that the safety and level of protection offered by a vaccine will remain unchanged when a new strain is inserted in an influenza vaccine. In fact, the U.S. approved four vaccines against H1N1 recently on the basis that they were a strain change to each manufacturer’s seasonal influenza vaccine. An editorial in the New England Journal of Medicine, however, cautions that “any association of uncommon adverse events with the vaccine cannot be ascertained in studies of this size.” Since there are occasional instances of unusual reactions in the case of seasonal influenza, there is a compelling need to conduct post-marketing surveillance.

Contrary to earlier predictions that two doses would be required to produce the desired immune response, the trials have provided evidence that a single dose is capable of producing a robust immune response in a majority of the volunteers. Considering the limited vaccine production capacity in the world, achieving protection with one dose will mean a doubling in the number of beneficiaries. Another way of stretching the vaccine supply will be by using an adjuvant -- a substance added to a vaccine to improve the body’s response. The World Health Organisation recommends the use of an adjuvant as it reduces the amount of vaccine to be used in every dose. The trial in the U.K using an adjuvant vaccine at half the standard dose was able to achieve a level of immune response comparable to a 15 microgram vaccine after three weeks. The trials have been able to measure the level of immune response. The next vital step will be to assess the duration of antibody response after vaccination. One caveat needs to be entered, although it is unlikely to have made any major difference to the results. Only volunteers uninfected by the virus were enrolled for the trials, which were conducted when the virus was widespread in the community, but the possibility of H1N1 virus infection prior to enrolment and its role in priming the immune system cannot be ruled out.

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