When World Health Organization officials ask vaccine manufacturers to start producing vaccine to fight swine flu in a few weeks, they will be taking a calculated risk.
Flu vaccine companies can only make one vaccine at a time: seasonal flu vaccine or pandemic vaccine. Production takes months and it is impossible to switch halfway through if health officials make a mistake.
While some countries like the United States may be better prepared than others — American officials say production for next year's vaccine batch is so advanced they're nearly finished — the situation for many other countries is unknown.
About 70 percent of the world's flu vaccines are made in Europe, and WHO is trying to figure out how many doses of seasonal flu vaccine remain worldwide before asking vaccine manufacturers to start pumping out pandemic vaccine instead.
'Never done this before'
"It's a big decision. ... We've never done this before," said Marie-Paule Kieny, director of WHO's Initiative for Vaccine Research.
Kieny said the decision would be like guessing which vaccine the world will need most, but that WHO would try to hedge its bets by tallying existing seasonal flu vaccine stocks.
"We are gambling whether or not we will have enough seasonal vaccine, but it will not be an 'either-or' situation," she said.
Vaccine makers can make limited amounts of both seasonal flu vaccine and pandemic vaccine — though not at the same time — but they cannot make massive quantities of both because that exceeds capacity.
The impending decision to make pandemic vaccine will also complicate matters for countries in the southern hemisphere, where the flu season is just starting.
WHO usually makes recommendations about which seasonal flu strains should go into next year's southern hemisphere flu vaccine in September. But if vaccine manufacturers are already making pandemic vaccine in the fall, that will mean fewer doses of flu vaccine for people in the southern hemisphere.
Regular flu kills between 250,000 to 500,000 people worldwide every year. So far, swine flu appears relatively mild, except in Mexico. Until experts know how contagious and deadly the virus is, they won't know how many people actually need to be vaccinated. If it remains mild, it's possible many people won't need a shot.
But if swine flu evolves into a more deadly strain, more shots may be needed to protect wider swathes of the population. Another possible complication is that the pandemic vaccine will be made using the strain available now; whether that will work if the virus mutates isn't certain.
At the moment, WHO has a bit of breathing space. The key component for the pandemic vaccine, the "seed stock" from the swine flu virus, will not be available for a few weeks.
WHO has already put major vaccine makers on alert they may be asked to switch to making pandemic vaccine soon. According to Chris Viehbacher, chief executive of Sanofi-Aventis, Europe's biggest vaccine maker, his scientists are "working around the clock" on preparations for making a swine flu vaccine for when WHO comes calling.
But they too are waiting for more information. "It is premature to forecast how many doses of swine flu vaccine we could produce," Viehbacher said, noting the company needs to see how much antigen, the active ingredient in a vaccine, is required for each shot. If all goes well, the first doses should be available several months after production begins.
In the U.S., the government hasn't yet decided what to order. "It would all depend on how the outbreaks and the emergency unfolds," said Dr. Ruben Donis of the Centers for Disease Control and Prevention, who is heading the U.S.' vaccine work.
Vaccine makers have a few options when it comes to making pandemic vaccine. They could add the pandemic strain to the regular vaccine, but adding that extra strain reduces the number of shots you can make. Manufacturers might also use adjuvants, components used to stretch a vaccine's active ingredient.
Recent work on H5N1 vaccines, the bird flu strain many thought would ignite the next pandemic, may also help. John Treanor, a vaccines expert at the University of Rochester said that adjuvants developed during H5N1 research may come in handy now.
He said public health officials were indeed facing a vaccine dilemma. "There is only a certain amount of capacity for vaccine manufacturing," he said. "One has to make some well-reasoned choices, sometimes based on incomplete information."