A woman gets a flu vaccination from a nurse. The Northeast and Mid-Atlantic are seeing increased flu activity. (Eduardo Verdugo/Associated Press)

Here’s a fighting tip for anyone who managed to duck the flu’s first punch: Watch out for the next one.

Many communities are experiencing an increase in flu, part of a second wave that is hitting some regions of the country particularly hard, health officials say. The most affected region is the Northeast — New England, New York, New Jersey and Connecticut — health officials said. Some parts of the Mid-Atlantic also are seeing increased flu activity.

Federal health officials say it’s common for an uptick in flu to take place in March and April. That often is caused by an increase in the influenza B virus, a different strain 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,” Finelli said.

Six states — Connecticut, Delaware, Maine, New Jersey, New York and Rhode Island — reported widespread influenza activity for the week that ended April 12, the most recent data available.

In the Washington, D.C., region, doctor’s offices and hospitals say they are seeing patients with flulike 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; so far in April, the hospital has had 16 patients test positive for influenza B.

At Washington Hospital Center, of 54 patients who had flulike symptoms, the two who tested positive were for influenza B, said William J. Frohna, chairman of the hospital’s emergency department. On Tuesday, he treated a young woman who “fit the picture of flu” — three days of fever, body aches, cough and a sore threat. But because she was not considered high-risk, she was not tested, he said. “I think she was a flu-
positive patient,” he added.

This season’s flu vaccine covers two strains of the influenza A virus, including the H1N1 “swine flu” responsible for the global pandemic of 2009, 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 this flu season, but influenza B viruses account for the largest proportion of viruses that are circulating now, according to the CDC.

While it may seem odd to get a flu shot in April, health officials say those who haven’t been vaccinated should try to get one. Consumers may have to call around because retailers may not have a ready supply, they advise.

The rest of the country is experiencing fairly normal flu activity for this time of year, according to CDC data. For the week that ended April 12, 1.5 percent of patient visits to doctors were for flulike illnesses nationally. But in New York, flulike illness made up 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, data show.

The second wave of flu typically goes away in May. Yet even into the summer, flu lingers.

Watch the spread of the flu across the U.S. each week.

“It never completely disappears,” Finelli said.