Metro

Flu activity in US rivals 2009 swine flu pandemic

Flu-like activity in the United States has reached the second-highest peak seen in the past two decades — just slightly below the record high — and flu-associated hospitalizations and deaths continue to surge, according to new numbers from the CDC.

The picture in Massachusetts is similarly bleak. Flu activity has reached the second-highest level in at least the past decade.

“I wish there were better news this week but almost everything we see is bad news,” said Dr. Anne Schuchat, acting director of the US Centers for Disease Control and Prevention.

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The latest statistics showed that the share of outpatient visits for influenza-like illness — defined as fever of 100 degrees or higher coupled with a cough, sore throat, or both — was 7.65 percent for the week ending Feb. 3.

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Since the 1997-1998 season, when flu data collection began, the only higher rate ever measured in a single week was during the 2009 swine flu pandemic when the proportion of outpatient visits for flu-like illness peaked at 7.71 percent.

Flu-associated hospitalizations also continued to rise in the most recent week of data.

“Overall, hospitalizations are significantly higher than what we’ve seen [in the past] for this time of year,” Schuchat said.

And because data about hospitalizations tend to lag, experts expect those numbers will grow.

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“We don't have any signs of hospitalizations leveling off yet,” Schuchat said.

In the most recent week, flu-associated deaths of 10 children were reported to the CDC, bringing the total number of pediatric deaths to 63 so far this season.

So if flu activity now is about as bad as it was in 2009, why doesn’t this season qualify as a pandemic?

That’s because the pandemic label only applies if the flu virus circulating is one that scientists haven’t detected before, making it particularly effective at spreading because people don’t have immunities built up. Federal health officials said they do not believe the virus circulating this season is new.

Among people of all ages, the CDC monitors how deadly the virus is by using estimates of the number of people killed by the flu and by pneumonia, a common complication of the flu.

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Those figures showed that in the most recent week of data — which is from the week ending Jan. 20 because of lags in reporting deaths — about one in 10 deaths in the country were attributed to either the flu or pneumonia.

“It’s a wake-up call about how severe influenza can be and how we can never let up our guard about this virus because it’s always changing,” Schuchat said.

In Massachusetts, it’s been a particulalry rough season. Since 2007, the earliest year of available state data, the state has seen higher rates of reports of flu-like illnesses during only one other stretch: the height of the 2009 swine flu pandemic. And the flu activity rate this year is just barely behind that peak.

The only good news: There was a drop in reports of people infected with Influenza A (H3N2), which is the type of flu that has infected the vast majority of people nationwide so far this season. But at the same time, reports of Influenza B infections are on the upswing, according to the CDC.

That different type of the virus can not only infect people who have managed to avoid the flu so far this season, but it can also reinfect those who came down with Influenza A earlier in the season — a cruel double whammy. It’s even possible for a person to be infected with both types of the virus at once.

Schuchat said it’s not uncommon for there to be second waves of Influenza B later in a season.

But, as always, it’s hard to predict whether flu activity will continue to increase in the coming weeks or if the worst may already be behind us.

“Flu is incredibly difficult to predict,” she said in a conference call with reporters Friday morning. “We don’t know if we’ve hit the peak yet.”

US Centers for Disease Control and Prevention

Matt Rocheleau can be reached at matthew.rocheleau@ globe.com.