2 varieties of flu seem to have disappeared during lockdown. Experts say it's possible they're gone.

• Two varieties of flu have not been spotted for over a year, STAT reported.
• Public-health measures to control the coronavirus pandemic may have led these strains to disappear.
• It's still possible, however, that they are circulating undetected.

It's been a lonely year to be an influenza virus.

Infectious-disease protections put in place during the coronavirus pandemic to protect people from COVID-19, such as mask requirements, school closings, and travel restrictions, made it nearly impossible for flu viruses to travel as well as they normally do.

The US, for instance, saw a 98% decrease in hospitalizations for flu in the 2020-2021 season. Rates in the UK dropped by 95%.

This was not just a result of sick people staying out of the clinic during the pandemic.

"There was a lot of flu testing happening - there was just not a lot of flu virus," Alicia Budd, a Centers for Disease Control and Prevention epidemiologist, told Insider during a recent Association of Health Care Journalists fellowship. "The peak - if you can call it that - was 0.4% of the specimens tested for flu were positive."

Normally during the height of flu season in the US, the share of positive tests ranges from 24% to 30%.

In fact, it's possible that the sharp drop in flu cases has been so dramatic this year that it killed off some versions of the flu. Two varieties haven't been seen for over a year, STAT reported. That could make it easier for vaccine makers to build a better flu vaccine - one targeted at a dwindling number of threats.

But experts caution it is still too early to draw any conclusions, in part because there were so few positive flu tests recorded last season.

"The actual number of specimens this year compared to previous seasons is extremely low," Budd said.

"It's obviously still important to know what viruses are out there," she continued, adding: "That helps us understand a little what we might be on the lookout for this coming season and if there are updates that are needed to the vaccine."

2 flu viruses have been missing from databases for over a year

The flu-virus family is split into two types: A and B. Type A is made up of subtypes, two of which account for most type A flu viruses in people: H1N1 and H3N2. Those are further broken down into clades. Type B influenza, meanwhile, is split into two lineages, B/Victoria and B/Yamagata, which are also subdivided into clades.

Since March 2020, there have been no reports of one particular clade of H3N2 (called 3c3.A), according to GISAID, a global database in which researchers register and track which flu viruses are circulating.

"I do think we're likely to lose a little bit of the H3N2 diversity," Richard Webby, a flu expert at St. Jude Children's Research Hospital, told STAT.

He said that'd be a "great thing" for the annual decision-making about which strains to target with vaccines - a process Webby takes part in each year.

"Currently when we sit down to make recommendations for vaccine strains, it's always the headache virus," he said of H3N2.

There have also not been any reports of flu viruses in the B/Yamagata lineage recorded on GISAID since March 2020, but experts think it's a less one to have disappeared.

"The Yamagata and Victoria lineage viruses have been in cocirculation for more than 30 years," Dr. Larisa Gubareva, a flu virologist at the Centers for Disease Control and Prevention, told Insider.

Even if certain versions of the flu are dwindling in certain areas, she added, "they could still be causing infections in areas with limited virological surveillance, or they could potentially adapt again to gain a competitive advantage."

"Flu viruses are constantly changing," Gubareva said. "It's not unusual to see new viruses, or even groups of viruses, emerge and then wane."

This is also a difficult time to draw definitive conclusions about which flu viruses are most prominent, since so few people got sick last year.

In a normal year, flu watchers at the CDC collect and genetically characterize thousands of flu viruses, Budd said. "This season," she said, "as of the beginning of May, we had just made it to 10."

Could this make it easier to create better flu shots?

If any flu variety is really gone for good, that would be good news for the scientists who make annual flu vaccines.

The World Health Organization meets twice annually to predict which viruses will be dominant the following season so countries can produce vaccines in time. (The most recent meeting happened in February for the Northern Hemisphere.) But its predictions are somewhat hit-or-miss, since the flu continues to circulate and change in unpredictable ways at a rapid rate. If a flu virus that is not well covered by the vaccine becomes more prominent than was predicted, even vaccinated people may not be totally protected.

This was the case during the 2017-2018 flu season, when three quarters of people who got the flu shot in the US were not covered against the most common strain of H3N2 that wound up circulating. The CDC estimates that about 61,000 people died from the flu that season, the heaviest influenza death toll in the US since the 2009 swine-flu pandemic.

Flu watchers like Budd, however, are hopeful that this year's shot might match circulating strains better. Out of the 10 flu viruses the CDC tracked down last season, four were H3N2s and four were B/Victorias. Those eight all match the genetic groups covered in this year's vaccine. Still, it's too soon to predict what will happen with the flu this fall and winter.

"We're going to have to wait and see, because flu is unpredictable," Budd said. "I certainly would expect, though, that as life returns more and more towards pre-pandemic ways, that respiratory virus circulation will likely return to something more what we're used to than what we saw this past year."