Swine flu vaccine strains grow very slowly, delaying vaccine production

Published 13 July 2009

The fastest-growing of all the wine flu vaccine strains tested so far grows only half as fast as ordinary vaccine viruses; if the current pandemic behaves like the last H1N1 pandemic in 1918, the next, possibly worse waves of infection could be long over by the time vaccine contracts are filled

This is not good news: Vaccine producers have hit a snag making vaccine against the swine flu pandemic. According to a report by the World Health Organization (WHO), the fastest-growing of all the vaccine strains tested so far grows only half as fast as ordinary vaccine viruses. Vaccine companies must fill pandemic vaccine contracts with countries such as Britain, Australia, France, and the United States before they can start making vaccine for the rest of the world -- notably developing countries. Marie-Paul Kieny, the WHO's head of vaccines, told a meeting in Geneva last week that if the pandemic vaccine virus grew as fast as ordinary strains, these contracts could be filled by November 2009.

That date applies only if the vaccine works at half the dose of ordinary vaccines, with one shot enough to induce the required immune reaction. If the usual dose and two shots are needed, the contracts cannot be filled until April 2010.

Debora MacKenzie writes that the slow-growing virus means these dates could be pushed back to January 2010 and June 2010 respectively. If the current pandemic behaves like the last H1N1 pandemic in 1918, the next, possibly worse waves of infection could be long over by then.

WHO says countries such as China and India expect to produce their own vaccine supplies, but smaller low- and middle-income countries representing 2.7 billion people have "no current access to H1N1 vaccine". The report was presented to a closed, extraordinary meeting last week of the Strategic Advisory Group of Experts on Immunization (SAGE), a panel of independent scientists who advise the WHO on vaccine issues.

Wenqing Zhang of the WHO's flu program told SAGE that five different "seed" viruses have been produced and evaluated. The two highest yielding strains, however, are growing only half as fast as the seasonal H1N1 viruses used to make vaccine for ordinary flu.

MacKenzie writes that flu viruses contain eight genetic elements. Vaccine strains contain two genes for the main surface proteins of the virus the vaccine is aimed at, plus six from a strain that grows well in chicken eggs, which is where most flu vaccines are produced. These vaccine strains are produced either by reverse genetics, in which the eight genetic elements are artificially combined using genetic technology; or by allowing the two viruses to mix naturally. WHO collaborating labs used both methods to produce seed strains for the pandemic virus to make sure there were no unforeseen problems with one or the other.

Yet both methods have performed equally badly, each producing one of the two fastest-growing strains. The other strains are presumably growing even more slowly. Vaccine companies are trying to tweak the vaccine strains to select faster growing ones, while WHO collaborating laboratories are feverishly generating new strains, which they hope will grow faster.