As the initial uproar over swine flu recedes, public health experts are turning a wary eye to the calendar, concerned that the next chapter may unfold during the traditional fall flu season.
Swine flu could return, they warn, possibly with a vengeance. No one yet knows how the virus might spread, or whether it could mutate into something more severe than the mild infections seen so far in the United States.
For now, scientists are ramping up efforts to develop a vaccine, which could be available in the fall along with the seasonal flu shot. But mass producing a vaccine requires a major investment of time and money. The trigger hasn't been pulled to roll one out for the H1N1 swine virus, whose mysteries the medical community is still unraveling.
It's also too early to know if the world has dodged a bullet.
Experts say swine flu could play out in several scenarios. It could simply fade away. It could bounce across the globe over the coming months. Or it could follow the pattern of the 1918 influenza pandemic, which did limited harm during its initial summer wave. The alarming death toll followed the re-emergence of the virus a few months later.
"Right now, you could probably go on one of the gambling Web sites and find people taking odds" on the scenarios, said Dr. John Sinnott, director of Infectious Disease and International Medicine at the University of South Florida.
"We're not going to know until next February whether this is successfully over," he added. "My concern is that this is going to fade, people are going to forget about it, and then they are going to get rudely brought back to reality next winter."
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Epidemiologists are looking to the Southern Hemisphere to see what happens next with swine flu, as seasonal flu season begins in the lower half of the world. North America is moving out of the most dangerous time of the year for flu.
Flu season tracks the cold weather months, when people congregate indoors. Summer's heat and humidity may act also as barriers to spreading the virus, said Dr. David Morens, senior advisor to the director of the National Institute of Allergy and Infectious Diseases.
No one can say yet how swine flu will evolve. Viruses are tiny particles, but they are amazingly diverse. They can mutate quickly, picking up genes from their hosts and swapping them with other viruses.
"Every pandemic and every epidemic is a law onto itself, and they all behave differently, so we'll see," Morens said. "You plan for the worst and hope for the best."
So far, scientists have not seen in today's H1N1 virus the genetic markers associated with the virulence of the 1918 influenza. There is no reason to expect a repeat of history in the fall, even if the swine flu virus circulates with seasonal flu strains.
But it's always possible that an influenza virus could emerge that poses even greater risks than the seasonal flu, which kills 36,000 annually in the United States.
There have been reports of "swine flu parties,'' in which people try to expose themselves to today's milder virus in hopes of protecting themselves against a stronger version. But that's not a good idea, according to public health experts, because much remains unknown about how this flu virus behaves and may evolve.
Meanwhile, they have to consider the worst-case scenario.
"It picks up some virulent genes and comes back like a lion and turns out to be the kind of pandemic we've all been dreading," said Dr. Glenn Morris, director of the University of Florida's Emerging Pathogens Institute. "The only way we'll find out is when it happens."
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U.S. Health Secretary Kathleen Sebelius said this week that the Centers for Disease Control and Prevention, the National Institutes of Health and the Food and Drug Administration are working together on an H1N1 vaccine. The viral strain has been identified and testing will begin soon. It will be up to the scientists to decide whether swine flu poses enough of a threat to make and distribute the virus, Sebelius said.
But production of a vaccine against seasonal flu has been moving forward, as usual. The process takes about 10 months and aims at having the vaccine ready in the fall, for the start of flu season. Sebelius said the process is being accelerated to "make sure we clear the decks, so if a decision is made to produce a vaccine for H1N1, we're ready to go."
The CDC is exploring adding the H1N1 strain to the seasonal vaccine, something Dr. Richard Besser, the CDC's acting director, called "an attractive approach," but not if it would delay delivery of the seasonal flu vaccine. Plus, health officials are well aware of the care that must be taken with vaccines. Besser has pointed to lessons learned from the swine flu scare of 1976, when only one person died from the swine flu itself, but hundreds of others were killed or seriously injured from complications from the vaccine.
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Health officials expect robust interest in flu vaccines this fall.
"I think that would be a good bet," said Dr. Carolyn Bridges, an associate director of epidemiologic science at the CDC's National Center for Immunization and Respiratory Diseases.
Bridges said the swine flu outbreak has been an opportunity to get the message out about the dangers of seasonal influenza. Public health officials say more Americans should be vaccinated, especially vulnerable groups such as the elderly and the very young, pregnant women, those with chronic illness and health care workers. About 64 percent of people over age 65 received the flu vaccine in 2006, a figure that has stayed mostly flat since the late 1990s, she said. Among children ages 6 to 23 months, 48.4 percent were immunized in 2006, the CDC estimates.
"I do anticipate that we'll have more interest this year in prevention of seasonal influenza," Bridges said.
Letitia Stein can be reached at firstname.lastname@example.org. Richard Martin can be reached at email@example.com.