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TORONTO - Swine flu vaccine production has hit a snag, with manufacturers reporting a disappointingly low yield when vaccines viruses are grown in eggs.
The World Health Organization says so far the yield for egg-based production is half or less what manufacturers get when they make vaccine to protect against seasonal H1N1 viruses. The lion's share of influenza vaccine is made by companies that grow the viruses in eggs.
New seed strains are being made in the hopes of increasing the vaccine yield, a report by the WHO's vaccine chief, Dr. Marie-Paule Kieny says.
But if the yield cannot be increased, it will slow the rate at which pandemic vaccine comes out of the production pipeline, adding to the time it takes to protect populations in countries like Canada that have purchased vaccine. And countries that haven't pre-ordered pandemic vaccine would face substantial delays before manufacturers have product to sell to them.
"There's nothing to suggest it will take longer to make vaccine, if in fact everything goes as planned. The question is: How much?" says Dr. Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota.
"There is nothing magical about making this virus. The questions will be: How much? When? and Where will it be available?"
The yield problem is revealed in presentations WHO staff made to last week's special meeting of the expert panel that advises the Geneva-based global health agency on vaccine issues.
The body - called the strategic advisory group of experts on immunization or the SAGE - was convened to give WHO counsel on a variety of questions about pandemic vaccine use. Those include which groups should be given priority when vaccine becomes available and whether the WHO should recommend companies use adjuvants, which are boosting compounds that could help stretch limited supplies.
Kieny, head of the WHO's initiative for vaccine research, was not available for interview Sunday. The WHO is expected to reveal details of the SAGE's deliberations and recommendations on Monday.
But a report to the meeting by Dr. Wenqing Zhang of the WHO's global influenza program says that vaccine manufacturers who use so-called wild-type viruses (unmodified viruses like those now circulating around the globe) are reporting yield rates similar to what they get when they grow seasonal H1N1 viruses in Vero cells, a cell culture medium. However, few manufacturers produce flu vaccine this way.
Most make vaccine in eggs, using a reassortant or hybrid seed strain designed to improve the chances of a good yield. These seed strains can be made by a couple of methods, but the end result is a hybrid with the external genes of the virus that vaccine is to protect against and the internal genes of a virus with a proven track record for growing well.
Zhang's presentation says that of the various reassortant vaccine viruses that have been made, the one with the highest output still only generates about half of the yield seen with seasonal H1N1 vaccine production.
Kieny's presentation calls the yield "less than optimal" and says laboratories in the WHO's lab network are generating new sets of vaccine viruses as quickly as possible.
Her presentation illustrates the impact low yield would have on availability of vaccine.
Somewhere between 850-900 million and 1.8 billion doses of pandemic vaccine are already spoken for, she reports. The low end of the scale represents what would be needed by countries with contracts if it is shown that one shot will be enough to protect a person; the high end represents what those countries would need if two shots per person are required.
If all manufacturers used the lowest possible effective dose, if yields are on a par with seasonal H1N1 production and if countries only used one dose per person, manufacturers could fill all their advantage purchase orders by mid-November, Kieny's presentation suggests.
That best-case scenario also requires that all manufacturing capacity remains devoted to pandemic vaccine and no portion shifts back to the production of seasonal vaccine for next year's Southern Hemisphere flu season.
If companies don't use low doses and countries that have pre-purchased vaccine demand two shots for all their citizens, it could be mid-April before the vaccine manufacturers in high-income countries have free capacity to devote to making vaccine for middle-and-low income countries, Kieny's presentation estimates.
Ninety per cent of the world's flu vaccine production capacity is in the high-income countries that use seasonal flu vaccine.
A lower yielding vaccine "would considerably push back the timelines," the presentation warns.
Assuming the yield is half that of seasonal flu vaccine production, it would be mid-January before producers could fill all contracts if they use a single-shot, low-dose regime, Kieny estimates.
She suggests even with low-dose shots, a low-yield scenario would mean manufacturers would not be able to fill all their existing contracts until next June if the countries opt for two shots per person for all their citizens.
-Follow Canadian Press Medical Writer Helen Branswell's flu updates on Twitter at CP-Branswell