The flu shot has been reducing the risk of infections in the United States by about half this season, according to reports issued Thursday by the Centers for Disease Control and Prevention, but the season is expected to continue for several more weeks.

The predominant virus strain this season is influenza A (H3N2) strain; the estimated effectiveness of the vaccine in preventing illness caused by that strain was 43%. The vaccine's estimated effectiveness against the influenza B virus was 73%. That amounts to overall protection of 48%, the CDC said.

The CDC's interim findings are based on data from November 28 through February 4 for 3,144 children and adults who are enrolled in the United States Influenza Vaccine Effectiveness Network. The CDC uses information from this network to routinely estimate effectiveness of the flu vaccine as the season progresses.

During the 2015-16 season, vaccine effectiveness was 47% -- about the same as this season. During the 2014-15 season, effectiveness was just 19%, according to the CDC.

"The viruses in the vaccine are a good match for the circulating viruses this season," said Brendan Flannery, lead investigator for the US Flu Vaccine Effectiveness Network. "The prediction for the H3N2 virus was right on in terms of that particular virus continuing to be a dominant virus."

As Flannery explained, the H3N2 was not seen much during last season, but it's the same virus that first appeared in the 2014-15 season. "This vaccine is a much better match for the circulating virus than the vaccine we had two years ago."

In a separate report, the CDC noted that flu activity in the US began to increase in mid-December, remained elevated through early February and is expected to continue for several more weeks. Influenza A (H3N2) viruses have predominated, though the CDC also identified influenza A (H1N1)pdm09 and influenza B viruses.

From October 2 through February 4, 38,244 specimens tested positive for flu virus in the US, according to the new report. Age was reported for 13,306 patients: 7.9% were younger than 4 years old, 30.4% were between 5 and 24 years old, 30.3% were between 25 and 64 years old, and 31.5% were 65 or older. Fifty-one of 54 US states and territories are experiencing elevated levels of flu and flu-like illnesses. There have been 20 pediatric deaths reported this season.

The CDC recommends that unvaccinated people who are 6 months old or older still get a shot this year.

"Influenza activity may have peaked where there were early peaks in the Northwest and on the West Coast," Flannery said. "Activity is increasing in some of the Midwest and the East Coast."

However, it's "unpredictable where it's going to peak," he said, and after what appears to have been a peak, "there can still be an increase in activity in some areas." For example, it seemed like a peak occurred in early January, but afterward, activity rose in several areas.

"We were a bit surprised by how much the influenza activity has increased recently," Flannery said, "and I think that strengthens the message for people to still get vaccinated."