As holiday travelers begin to crisscross the country, public health officials warn that swine flu cases could surge back from a recent decline, and they caution that flu season is far from over.
State officials say the number of patients with flulike symptoms, who represented 12 percent of all emergency room visits at a high point for infection in late October, has since declined substantially. But the officials say that even if this wave has peaked, roughly half the people who are going to get sick from the flu haven't yet. Precautions, such as hand washing and keeping those who are sick at home, should continue - even if that means missing Thanksgiving dinner with family.
"We are encouraging people to use as much common sense as they can," said John M. Colmers, state secretary of health and mental hygiene. "Recognize that your actions not only influence yourselves and loved ones, but many other people as well."
The state is making progress on vaccinating target groups, with just over a million doses of the H1N1 vaccine ordered. That's still far short of the estimated 2.9 million people in priority categories that should receive a dose, including pregnant women, children, those with underlying health conditions, those who care for infants and health care workers.
Epidemiologists don't know why the H1N1 virus appears to have peaked, especially now, a time of year when flu season typically ramps up.
"Now our temperatures are getting into the kind of climate conditions that influenza likes - it's a bit of a paradox," said Dr. William Schaffner, chairman of preventive medicine at Vanderbilt University and an expert on infectious diseases.
Experts have a few theories about what could be at play. The vaccine appears to offer good protection against the flu. And since the virus has been circulating since the spring, many people have already been infected. Combined, those factors mean the virus has fewer susceptible people to infect.
"Herd immunity is an important concept," said Dr. Trish Perl, a Johns Hopkins Hospital epidemiologist. "What we know is that communicable disease can go through populations as long as they are susceptible to it. There is some threshold; we don't always know if that is 95 percent or 75 percent, but once that percentage is immune, transmission of the organism can no longer be sustained."
It's also why some doctors believe that older people might have some protection against H1N1 flu - they were likely exposed to similar strains of flu decades ago. In fact, early results from H1N1 vaccine trials showed that even younger groups - 11 years and older - had some background immunity, said Dr. Karen Kotloff, who led trials at the University of Maryland's Center for Vaccine Development.
But any herd immunity that has developed can be interrupted when people travel and interact with new people, which is why public health experts are telling the public to continue to be cautious.
So far, 30 people in Maryland have died from swine flu and 787 have been hospitalized. Nationally, about 3,900 people have died and 98,000 people have been hospitalized. Millions have been sickened, according to estimates from the U.S. Centers for Disease Control and Prevention.
Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, said last week that the nation is beginning to see a decline in flu cases in many states. She also warned that holiday travelers could spread more disease and urged anyone sick not to leave home.
"All the kids get together with their grandparents and that's a lot of exchange of warmth and love, but a little exchange of viruses, too," she said. "So, we don't really know what's going to happen with Thanksgiving. But we think it's critical, if you're sick, to stay home."
At Hopkins, the swine flu cases peaked in late October and the decrease since then has been dramatic, Perl said.
From the beginning of September through Nov. 14, the hospital had 529 H1N1 cases. About 120 of them came in the week of Oct. 24 alone. Last week, the hospital had five cases. So far this week, just one.
While many of the earlier patients were children, who are more susceptible to this new flu, the week of Nov. 14, all the cases involved adults, Perl said. Of the total cases, about two-thirds have been outpatients and the other third have been people hospitalized for the virus, she said. The hospital isn't testing everyone, and Perl warns the figures likely underestimate the cases.
Infectious disease experts say while the apparent peak of the virus is good news, the future is unpredictable. There are a few possibilities: A third wave of H1N1 flu could begin once the weather gets colder. Or H1N1 could fade while seasonal flu cases rise, which typically happens in December. Or, in an unlikely scenario, no winter flu season materializes at all.