(CNN) 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.

Go to CNN.com/unseenenemy for more stories and resources about epidemics, pandemics and the CNN Films documentary, "Unseen Enemy."

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."

Building a vaccine

The flu shot is an inactivated influenza vaccine. Each February, the Advisory Committee on Immunization Practices, a group of medical and public health experts, makes the final decision about which virus strains will go into vaccines, which will be sold beginning in September to protect people during the forthcoming flu season.

The committee's decision is based on information from more than 100 countries, where influenza-monitoring centers conduct surveillance of circulating viruses. The committee members consider which viruses are making people sick, where those viruses are spreading and how well the previous season's vaccine protects against them.

After the committee selects the strains to be used for the vaccine, manufacturers will produce trivalent formulations with three strains and quadrivalent formulations with four strains for the upcoming season; both trivalent and quadrivalent shots are equally recommended, according to Flannery.

Generally, vaccine effectiveness varies and not only based on what type of viruses are circulating.

"How well the vaccine matches the circulating virus is certainly a big factor," Flannery said. "But that's not the whole story, and sometimes there are differences by age or ... the immune status of the individual. So there are individual factors as well as the circulating viruses that influence vaccine effectiveness."

The pattern this season is very similar to previous H3N2 seasons, according to Flannery, with higher hospitalization rates seen in the 65 and older population. "Young kids also are having severe illness," he said.

"Right now, it looks on track to be a moderately severe H3N2 season," he said. "It seems like it's similar to the 2012-2013 season, which was a season with millions of cases and about 60,000 hospitalizations and 2,000 deaths."

'If you haven't gotten vaccinated ... run'

Infectious disease specialist Dr. William Schaffner freely admits that "the flu vaccine is an imperfect vaccine. It often cannot prevent flu completely."

"But if you get vaccinated and get the flu, your disease is going to be in all likelihood much milder than it might have been. You won't have to be admitted to the hospital, you won't have to go to the ICU, and you won't die."

"So half a loaf is better than none," Schaffner said. "In the meantime, we support the people who are working in laboratories to make an even better influenza vaccine, which we hope will be available a few years down the road.

"Still, we can do a lot of good with a pretty good vaccine available today."

With flu continuing to occur through this month and into March, there's still flu that can be prevented or made less severe, Schaffner said. "So that's why we always say it's not too late -- but it is very late.

"If you haven't gotten vaccinated and you haven't gotten sick yet, run -- do not walk -- this afternoon. You should get your flu vaccine, because it does take 10 days to two weeks for maximum protection."

Join the conversation See the latest news and share your comments with CNN Health on Facebook and Twitter.

If you do come down with the flu, it is important to call a health care provider who can give you a prescription for an antiviral drug.

"And what that will do is also make the disease less severe, and you're likely not to be ill for as long," Schaffner said. "It shortens the duration of the influenza, and it also makes it less likely you will spread that virus to the people around you."

Specifically, the CDC advises early treatment with oseltamivir for anyone at risk for suffering flu complications, a group that includes "65 and older, young children, people with underlying compromising health conditions" and those who are hospitalized, according to Flannery.

"If influenza is suspected, it doesn't have to be confirmed," Flannery said. "It should be started as early as possible and shouldn't depend on whether someone was vaccinated or not."