A second wave of the flu is going around Massachusetts.
Early on, the flu season was dominated by one type of the virus, influenza A.
But while type A has been on a steady decline in recent weeks, another version of the flu, type B, has been surging, according to data released Friday by the state Department of Public Health.
That’s led to a spike in flu-activity overall across the state, though the number of cases in the past few weeks has still remained below peaks seen in mid- and late-February.
“There’s been a little bit of an uptick in the flu in the past few weeks, and most of it has been influenza B,” said Dr. Larry Madoff, director of the state health department’s Division of Epidemiology and Immunization.
Type A cases peaked this season in the week that ended on Feb. 11, when there were 817 laboratory-confirmed cases statewide. The number of type A cases has dropped virtually every week since, falling to 231 in the week that ended on April 1, state data show.
Meanwhile, during that same span, the number of laboratory-confirmed type B cases statewide has risen virtually every week, from 51 to 349.
(Laboratory-confirmed figures likely underestimate how many people actually contract the flu, but are a good indicator of overall trends.)
The trend has been similar within Boston, according to data from the city’s health commission. And at the national level, there’s also been a spike in type B cases of the flu.
Madoff said it’s not unusual for type B to peak around this time of year, and overall, this flu season has been fairly typical in terms of timing and intensity.
Within the broader type A and B flu categories, there are even more specific varieties of the flu, which are what actually circulate and vary from year to year.
Flu vaccines are made to protect people against those more specific flu versions of both influenza A and B.
Each year, months before the start of the flu season in the United States, health authorities examine which specific versions of the flu are circulating elsewhere in the world and make their best judgment about which are likely to strike and include them in that year’s vaccine. It can take several months to produce large quantities of influenza vaccine.
Vaccination can reduce the risk of contracting the flu by as much as 50 to 60 percent, recent studies have shown, according to the Centers for Disease Control and Prevention.
Madoff said this year’s vaccines have been effective, and recommended people who haven’t been vaccinated already to do so. The season should be winding down, but elevated flu activity could linger into May, he said.
“If you haven’t gotten one yet, you should still get one,” said Madoff.
The flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs, the CDC says. Symptoms can include fever or feeling chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, and fatigue.
It can cause mild to severe illness, and can lead to death, officials say.
Each year, the flu sickens millions of people in the United States, sending hundreds of thousands of them to hospitals, and thousands, if not tens of thousands, die, the CDC says.
People most at risk for serious complications include young children, pregnant women, people 65 years and older, and people with chronic medical conditions, such as asthma, diabetes, or heart disease.