Here's a tip for those who managed to duck the flu's first punch: Watch out for the next one.
Federal health officials say it's common for an uptick in flu to occur in March and April. Often, that is caused by an increase in the influenza B virus, a strain different from the ones that dominated earlier in the flu season.
"We are experiencing a second wave of flu, and that second wave is influenza B," said Lyn Finelli, chief of influenza surveillance and outbreak response at the U.S. Centers for Disease Control and Prevention. "In some communities in the Northeast, they tell us that they're experiencing more influenza now than during the peak of the flu season in late December and early January."
Six states -- Connecticut, Delaware, Maine, New Jersey, New York and Rhode Island -- reported widespread influenza activity for the week ending April 12, the most recent data available.
In the Washington, D.C., region, doctors and hospitals say they are seeing patients with flu-like illnesses, some of whom are testing positive for influenza B. George Washington University Hospital has had a slight increase in influenza B cases in the past month and a half, said spokesman Steven Taubenkibel. In March, 11 patients tested positive for influenza B; this month, the hospital has had 16 patients test positive for the illness.
This season's flu vaccine covers two strains of the influenza A virus, including the H1N1 "swine flu" responsible for the 2009 global pandemic, and two strains of influenza B. All four strains were included in the quadrivalent vaccine available for the first time this season. The H1N1 virus has dominated during this flu season, but influenza B viruses now account for the largest proportion of viruses that are circulating, according to the CDC.
Although it may seem odd to get a flu shot in April, health officials said those who haven't been vaccinated should do so. Consumers may have to search because retailers may not have a ready supply.
The rest of the country is experiencing fairly normal flu activity for this time of year, according to CDC data. For the week ending April 12, 1.5 percent of patient visits to doctors were for flu-like illnesses nationally. But in New York, flu-like illnesses accounted for more than 4 percent of all visits, increasing slightly over the previous week. In New Jersey, those illnesses accounted for more than 3 percent of visits, also edging up from the previous week, the data show.
The second wave of flu typically ends in May. But even into the summer, flu still lingers.
"It never completely disappears," Finelli said.