"Vaccine manufacturing is not an easy process," laboratory chief Wayne Pisano told reporters Wednesday on a visit to Sanofi's Swiftwater, Pennsylvania facilities.
Inside the new, sterilized, bunker-style building, visitors are made to wear blue gowns, glasses, head covers and three layers of shoe covers before they can watch a syringe inject the H1N1 strain inside the white eggs as they drift by on a moving mat.
The strain takes several days to develop inside the incubating eggs, and three months are needed before the vaccine is ready for shipping. During that time the egg contents have to be purified, the virus made inactive and submitted to numerous quality controls.
The facility, operating non-stop round the clock, is the only one producing H1N1 vaccine injections in the United States, where the pandemic is estimated to have affected millions of people and killed more than 1,000.
A division of the Sanofi Aventis pharmaceutical group, Sanofi Pasteur has boosted its staff from 1,100 to 1,300 to meet the US government order for 75 million injectable doses of the H1N1 vaccine. Other manufacturers supply nasal spray vaccines.
US health authorities have recently acknowledged greater shortfalls than anticipated in the vaccine supply, as long queues form outside authorized clinics and health centers in the innoculation drive.
"You can't master all the elements because that's the nature of the beast. You develop a certain experience as you go along," Sanofi Aventis director general Chris Viehbacher told AFP. He admitted initial vaccine production was weak.
Cultivating and growing new virus strains inside chicken eggs is a slow process. It takes several days to create optimal conditions for the incubation stage alone.
Initial vaccine yields were 70 percent smaller than those for the seasonal flu, but now they are about the same, Pisano said.
Shipment delays are now history, and production is expected to double next month from five to 10 million doses per week, the laboratory chiefs said.
And besides the difficulty that the biological process itself poses, the number and strictness of the controls the vaccine must pass add to production time.
"It's important that the vaccine is effective, it's even more important that it is safe," said Pisano.
Another laboratory head Susan Powers explained that the priority given to H1N1 necessarily eliminated a number of steps in the development process.
"Under normal circumstances, you might wait until you get 10 samples to test the vaccine, but right now we don't wait," she said.
Swine flu has infected as many as 5.7 million people in 48 US states since it first broke out in April. Of the deaths in the United States, at least 129 were children, according to data from the Centers for Disease Control and Prevention (CDC).