Your burning swine flu questions, answered.

Your burning swine flu questions, answered.

Your burning swine flu questions, answered.

Answers to your questions about the news.
May 1 2009 6:36 PM

Swine Flu FAQ

Will getting swine flu now protect me later? Why is winter flu season? Why do vaccines take so long?

Pig. Click image to expand.
A pig is washed at a farm in Indonesia

The World Health Organization upgraded its swine flu alert status from 4 to 5 on Wednesday, declaring the outbreak a full-blown pandemic. As of Friday, there were 141 confirmed cases in the United States and 331 cases worldwide. With the rising numbers come new questions:

If I get swine flu this year, am I more likely to survive a more serious outbreak next year?

Yes. Getting sick with the flu—or even just being exposed to it—is a lot like getting vaccinated. It doesn't guarantee future immunity, but it does lessen the chances of getting sick again. Vaccines, however, are a more reliable way to fend off sickness, since they are designed to protect you against future strains. (Scientists can predict how the virus will evolve and design vaccines.) Getting sick now, meanwhile, merely protects you from the virus's current incarnation.

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There are no hard numbers for how often exposure grants immunity, but if vaccination success rates are any indication, the odds will vary by season and by strain. Flu vaccines typically succeed—i.e., they prevent you from getting the flu—between 70 percent and 90 percent of the time. In 2007, however, the rate went down to 44 percent.

What about seasonal flu? If I'm exposed to that, does that help ward off swine flu?

No. While past studies have shown that seasonal flu vaccines may help ward off avian flu, the Centers for Disease Control and Prevention announced last week that they don't work against swine flu.

Why is flu season in winter?

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Because the virus is more stable and stays airborne longer when it's cold. Cells infected with flu virus are coated in a fatty material that hardens and protects them in low temperatures. (When you inhale a flu particle, that coating melts in the respiratory tract, releasing the virus.) Viruses also stay in the air more easily when the air is cold and dry. When it's humid, by contrast, flu-carrying vapor droplets get heavier and fall to the ground.

Temperature makes a huge difference in transmission rates. In a study published in 2007, flu researchers exposed guinea pigs to a virus at various temperatures. They found that transmission levels were high at 41 degrees Fahrenheit but declined at increased temperatures. At 86 degrees, the virus did not transmit at all. (Slate's Andy Bowers tackled this question for NPR in 2003.)

The last major swine flu outbreak was in 1976. Where has swine flu been since then?

In swine. But the question is misleading, since the current swine flu is not the same swine flu that broke out 30 years ago. Yes, their structure is similar—they're both H1N1—but they are different viruses. What we currently call "swine flu" probably originated after 1976 and only recently became capable of transmission from pig to human and human to human.

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Federal officials said it could take till January to produce enough swine flu vaccine to protect every American. Why does it take so long?

Because we rely on half-century-old technology. Scientists first have to decide which flu strains to use in the vaccine—they typically pick three, based on predictions of how the virus will evolve by next year. (It's still unclear whether they'll include the swine flu in next year's seasonal flu vaccine.) They then inject the strains into chicken embryos, wait for them to multiply, and extract them several days later to create a so-called "seed stock." Then comes testing on lab animals, followed by any necessary adjustments to the stock. (Haste can have a price: During the swine flu outbreak of 1976, 500 people who got vaccinated developed a disease called Guillain-Barré syndrome, and 25 of them died.)

Once researchers have finalized the vaccine, they repeat the embryo injection process millions of times. So even if they finalize the vaccine in June, it will take at least until September to produce the first 50 million or so doses. Producing all 600 million doses—enough to give Americans the usual two shots—would take months longer.

A new experimental method would grow the virus in individual cells instead of eggs. That process is a lot faster—it could shave weeks off the time it takes to create the seed stock—since cell cultures grow faster than eggs. But that method hasn't been approved yet by the FDA, and there are still doubts about its effectiveness.

Got a question about today's news? Ask the Explainer.

Explainer thanks John M. Barry, Robert Krug of the University of Texas at Austin, and Trish Perl of Johns Hopkins University.

AP video: Government Prepares for the Worst