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Health authorities say this year’s influenza vaccines appear to be a “very good match” to the strains currently circulating around the country so far this season. The new data comes as officials are redoubling their calls for Americans to get vaccinated as this year’s flu season gets off to an early and worrisome start.

“We look in real time as to how well we think the influenza match is to what’s circulating. And right now, the good news is that it looks like it is a very good match,” Centers for Disease Control and Prevention Director Dr. Rochelle Walensky told reporters at a briefing on Monday.

Every year, the CDC tests hundreds of samples collected from Americans with the flu, in an effort to track if the virus could be evading the current season’s vaccines.

The CDC reported on Friday that most of the viruses tested so far this season are similar to the strains that were picked out by officials for this year’s updated vaccines. Using antibodies from ferrets, the agency said almost all samples they tested “were well recognized” by the animals’ immune responses.

Almost every sample tested by public health laboratories around the country so far this year have been from the Influenza A type of virus. Only 0.1% were Influenza B. All viruses tested so far this season have also been susceptible to the four antiviral treatments that are currently recommended for flu patients.

However, actual estimates of how well the vaccines are fending off flu cases and severe disease will have to wait until the spring, when results begin to be released from the CDC’s ongoing vaccine effectiveness studies.

Last March, the CDC concluded that last season’s “influenza vaccination did not significantly reduce the risk of outpatient medically attended illness” against the predominant strain of viruses.

“We would expect clinical vaccine effectiveness to be quite good, but we won’t know that until sometime in the early part of next year, depending upon how these vaccine effectiveness studies enroll,” the CDC’s Tim Uyeki told a webinar hosted by the Infectious Diseases Society of America on Saturday.

While the flu season started far earlier this year compared to pre-pandemic seasons, Uyeki cautioned that it could still be far from over. Influenza B viruses tend to start increasing after Influenza A peaks, he said, and could lead to a renewed wave.

“So just because we haven’t seen a lot of influenza B so far, it may be coming, if this is a typical influenza season,” said Uyeki.

At the CDC briefing on Monday, the American Medical Association’s Dr. Sandra Fryhofer urged Americans not to let down their guard even if they had recovered from a flu infection already once this season. She urged Americans to seek out a flu shot rather than risk getting infected again.

“We’ve forgotten how bad the flu can be. But this year’s season is a shout out that it can get really bad and it’s here, so people need to get vaccinated,” Fryhofer said.

Weekly CDC surveys suggest that about 40% of children have gotten their flu vaccination this year, around the same rate as this time last year.

However, early data from shots given in pharmacies and doctors offices suggests the rollout may be lagging in adults. More than 51.7 million shots were given through mid-November to adults there this year, compared to 54.1 million for the same point in 2021.

Flu hospitalizations have also now begun to accelerate around the country this year, officials said, with the pace of new admissions having “almost doubled” around Thanksgiving compared to the week prior. The weekly rate of hospitalizations tracked in the CDC’s Influenza Hospitalization Surveillance Network has already reached the peaks that were seen several weeks later in some pre-pandemic winters.

And the pace of new COVID-19 hospitalizations has now begun to accelerate as well around the country, increasing around 17.4% compared to the week prior.

“The rise in cases and hospitalizations is especially worrisome as we move into the winter months, when more people are assembling indoors, with less ventilation, and as we approach the holiday season, where many are gathering with loved ones across multiple generations,” said Walensky.