This has been a flu season like few others.
Normally at this time of year, influenza is rampant in the U.S., prompting hundreds of thousands of people to stay home in the dead of winter with fever, aches and pains.
Now, after raging through college campuses and communities last summer and fall, cases of the new H1N1 swine flu virus have dwindled to a trickle, and run-of-the-mill seasonal flu has barely made an appearance. Not one state reported widespread flu illness to the Centers for Disease Control and Prevention for the week ended Feb. 20, the latest data available. The percentage of all doctors' visits by patients with influenza-like symptoms has dropped from a high of 7.8% in late October—the largest peak since the agency began surveillance in 1997—to 1.8% in late February, well below the norm for flu season.
Doctors and flu experts say the lull is unusual. "This is typically the peak of flu," said James Turner, executive director of the University of Virginia's department of student health. He said the Charlottesville, Va., student health center usually sees as many as 130 students a week complaining of flu symptoms this time of year. Recently, no more than three to five students a week have been coming in with fever, cough or other signs of flu, he said.
It is not clear why there is so little flu, particularly swine flu, going around, experts say. "Surely there's a sufficient number of people who haven't been infected or vaccinated," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
Many scientists say the answer probably has to do with how the flu virus progresses. Influenza comes and goes in waves, normally running from October through May. But pandemic viruses—new viruses that emerge and spread quickly around the globe—often move to a different rhythm, and can reach their busiest stage at unusual times like summer and early fall, although the reason for this isn't understood. Flu has peaked in late February or early March in 20 of the past 26 flu seasons, said Lyn Finelli, the CDC's chief of flu surveillance and outbreak response. But the latest swine flu wave started in August, and peaked in late October, before waning.
Widely publicized preventive measures such as hand washing, and the large national vaccination effort thus far, may also have played a role in the sharp drop off in H1N1 infections, but it is not clear by how much, said Anthony Fiore, a medical epidemiologist with the CDC's influenza division.
By mid-January, swine flu had sickened about 57 million people in the U.S. and killed roughly 11,700—mostly young adults and children—from the time it emerged in late April through mid-January. Young people who ordinarily fight off flu easily ended up in intensive care units on advanced life support. Pregnant women, children, and other people at risk of flu complications stood in line for hours for vaccine, as slow production and delays prompted limited supplies.
In past years, when seasonal flu was the big worry, influenza has typically sickened about 25 million people a year, and directly killed about 8,000, according to the CDC.
The H1N1 flu virus hasn't disappeared, and another wave could still come this winter. "We've had peaks [of flu] as late as May, so we're not out of the woods yet," said Dr. Fiore of the CDC's influenza division. The virus could also mutate, spawning more new infections, though it hasn't done so yet. Flu experts estimate that more than half the country's population may still be vulnerable to the H1N1 virus, either because they haven't been sick or gotten vaccinated.
Some recent statistics could be warning signals. A few states have reported small increases in the percentage of doctors' visits by patients with flu-like symptoms. And the American College Health Association reported for the week ending Feb. 19 its first uptick in swine flu cases on college campuses since mid-December, to 4.1 cases per 10,000 from 2.7 cases per 10,000.
Dr. Turner, who is also president of the ACHA, said the organization plans to decide along with the CDC after getting more data this week whether to warn colleges to promote prevention measures to thwart a new wave that may be on its way. He said he is concerned that college students traveling on spring break could bring the disease back with them to campus. "If there's anytime we're going to see an increase in disease activity, I'd say March or early April is the time," he said.
What's expected for the future of the H1N1 swine flu virus.
- Flu experts say it is impossible to know if or when there will be another wave of swine flu.
- The flu season may not be over for this year, and swine flu could still return in strength. At least half the population remains susceptible to the flu.
- Some scientists expect any return of the virus won't be as severe as last fall. Millions of people are immune now, either because they were sick or have been vaccinated.
- Vaccine for H1N1 swine flu is still being administered, and next year's shot will include protection against the strain, along with two other seasonal viruses.
- Another possibility: The H1N1 virus could mutate and make the current vaccine, as well as next fall's version, less effective.
U.S. and global health officials have recommended that the new H1N1 virus be included as one of three strains in next season's flu vaccine. The CDC's vaccine advisory panel also expanded its recommendation for who should get flu shots to include young and middle-aged adults, among whom death rates from the new H1N1 virus have been higher than for the elderly or children. The change means that all Americans now will be encouraged to get flu shots when the next vaccine is ready in September.
The unusual course this flu season is taking isn't one that public health officials are eager to repeat. Not knowing when the next flu outbreak could occur will mean staying on high alert for much of the year. It would also mean another rush to get vaccine distributed quickly. The CDC, which normally starts tracking flu hospitalizations and deaths in October, will continue to monitor them this spring, summer, and early fall. "We're going to make sure when it hits, we know about it and can describe it," Dr. Finelli said.
Nancy Cox, the CDC's influenza chief, told a vaccine advisory panel last week that the pandemic H1N1 virus is likely to eventually become a seasonal strain, though it is not one yet. Globally, the pandemic virus is still circulating in tropical countries and some parts of Europe, but is waning in most places, according to the World Health Organization.
Flu experts say they don't know why there is so little seasonal flu circulating this year. One theory doctors are debating is that people infected with H1N1 swine flu may produce a chemical in their bodies that protects them against other flu viruses.
The new virus doesn't appear to have completely muscled out other flu viruses, as some past pandemic viruses have. While one seasonal strain appears to have succumbed to the new tide of H1N1, another that is considered particularly hard on the elderly—H3N2—has continued to circulate in pockets of the globe, albeit in small amounts.
While relatively few people are currently seeking out H1N1 shots, the CDC isn't giving up on promoting the vaccine this winter. About 155 million doses of H1N1 vaccine have been distributed so far this flu season, and about 86 million people have been vaccinated—a few million less so far than normally get vaccinated for seasonal flu. The agency is especially urging parents with children under 10 who have only had one dose to get a second, which it says is needed for full protection.
Even if a new wave of H1N1 illness occurs, some experts believe it won't be as large as the one last fall.
That's because millions more people have become immune either through infection or vaccination. "I think we'll have scattered outbreaks in communities that didn't have it," said Dr. Finelli.
Write to Betsy McKay at firstname.lastname@example.org