The rest of the world may be exhaling at the apparent easing of a potential swine flu pandemic, but some global experts are tempering their optimism with concerns about what one calls "the fall question."
That's the uncertainty over whether the current outbreak is only a preview of what's to come, an echo of previous epidemics — including the 1918 flu — that saw mild first cases of infection in the spring followed by more severe second and third bouts in the fall and winter that brought widespread infection, illness and death.
"Right now, you have to wait and watch," said Ann Marie Kimball, professor of epidemiology and an expert in emerging infectious disease at the University of Washington in Seattle.
But Kimball and others, including Dr. Margaret Chan, the World Health Organization’s director-general, warn that it's possible the swine flu could re-emerge in a stronger form next fall.
"We hope the virus fizzles out, because if it doesn't we are heading for a big outbreak," Chan told Britain's Financial Times on Monday. "I'm not predicting the pandemic will blow up, but if I miss it and we don't prepare, I fail. I'd rather over-prepare than not prepare."
More than 20 countries have reported cases of swine flu, with most of the more than 1,600 confirmed cases in Mexico, the United States and Canada. Health officials in the U.S. and abroad say that the death toll, which now stands at 30, is not as bad as they originally feared.
Flu season is ending in the northern hemisphere, where most of the current infections of the H1N1 swine flu virus have been reported. The coming summer with warm temperatures and higher humidity means that the flu will be less transmissible there, although it raises worries for the spread of virus during the approaching winter in the southern hemisphere. So far, countries there have escaped the outbreak.
But the big fear is that the current swine flu virus could die down now in the north, but mutate during the ensuing months to become more transmissible and more virulent, fueling a serious outbreak when flu season resumes in that hemisphere, Kimball said.
“You have to remember that you are dealing with a flu virus,” she said. “They do reassort, shift and drift.”
1918: Second wave most deadly
The echoes of the 1918 flu are unavoidable. Historical accounts long have suggested that the first pandemic wave appeared in the U.S. in the spring of 1918, causing illness, but no appreciable deaths above normal.
Dr. Jeffery K. Taubenberger, an influenza investigator in the Laboratory of Infectious Diseases at the National Institutes of Health, said cases may well have turned up in the spring, but evidence suggests they certainly did by the summer of that year, causing out-of-season infections and deaths among young, healthy people.
Those early cases were followed by far more fatal second and third waves in the fall and winter of 1918 and 1919 in the United States.
"When it got cool enough to spread well, the virus exploded," Taubenberger said.
The thought was that the virus initially was strong enough to cause serious illness in some places, but not fit enough to launch a pandemic. Quickly, however, it progressed from causing uncomplicated influenza infections to fatal pneumonia, eventually leading to an estimated 650,000 deaths in the U.S. and between 25 million and 100 million around the world.
That model has led some scientists and ordinary people to question whether coming down with a mild case of swine flu now might actually protect against more serious illness in the future.
So far, government health officials say they still can’t tell whether infection now would confer immunity, said Dr. Anne Schuchat, a deputy director with the federal Centers for Disease Control and Protection. And Taubenberger said flu viruses mutate so quickly, there's no guarantee that this virus will be the same in six months.
Other flu experts said they would recommend avoiding exposure because the virus is still capable of causing severe disease, as seen in the cases in Mexico. And few would be willing to take chances with their own families, said Dr. Anne Moscona, a professor of pediatrics, microbiology and immunology at Weill Cornell Medical Center/New York Presbyterian Hospital.
“I think that it’s just too unpredictable to know how this virus is going to be in any one individual,” said Moscona, whose own children are 8 and 14. “I would by far prefer to avoid it.”
Avoiding the swine flu outbreak, even as it wanes in this country, depends on the habits that health officials have been emphasizing: good hand hygiene, avoiding close contact with sick people, staying home at the first sign of illness.
It does not depend on stockpiling antiviral medications, such as Tamiflu and Relenza. In Miami, CVS pharmacist Lauren B. Strzelecki saw a several months’ supply of Tamiflu disappear overnight from her as patients with valid prescriptions but no symptoms of illness loaded up on the drugs.
“They said, ‘I just walked in and said I was really nervous about swine flu,’” Strzelecki said.
Drug-resistant antivirals a worry
Increased use of antiviral medications raises concerns about growing drug resistance, said Moscana and Dr. David M. Weinstock, a flu expert and an assistant professor at the Department of Medicine at Harvard Medical School.
Before this new strain of swine flu emerged, the dominant strain of H1N1 influenza was almost entirely resistant to oseltamivir, or Tamiflu. So far, the new strain is susceptible to Tamiflu and to zanamivir, or Relenza.
“What we’ve learned about influenza is every time we feel confident with the way it evolves, it throws us a curveball,” Weinstock said.
Although the new flu is not likely to become resistant overnight, ill-advised use could speed up the process, allowing flu viruses to develop immunity to the only drugs that remain.
“The worst case scenario is that if people decided they want to take their Tamiflu, they’d take one pill a week for the next four months, then get the flu,” he said.
That could accelerate the person’s own resistance to the drug, as well as the wider resistance in the larger community.
Worries about a second wave of flu, early immunity and growing drug resistance all could be eased by the development of a safe, effective, widely available vaccine to protect against the new H1N1 swine virus.
Federal officials have started growing enough “seed stock” of the novel virus to provide material to vaccine manufacturers, according to press briefings. So far, it appears that the process is going as it should.
“I’m very hopeful that we’ll have a vaccine by the fall,” Weinstock said.