COVID-19 is responsible for all but eliminating the flu this season.

And no one seems more surprised about it than infectious disease experts like Dr. Richard Martinello, associate professor of internal medicine and pediatrics at the Yale School of Medicine, who responded succinctly Friday when asked how many cases of the flu he’s seen lately.

“That’s easy,” he said. “Zero.”

In late summer and fall, public health officials crossed their fingers in hopes of a light flu season, given the demands the COVID-19 pandemic was putting on hospital resources. The threat of a “twindemic” of flu and coronavirus disease at the same time was ominous.

Now, well into the flu season, the worries have proven more unfounded than anyone could have predicted.

“We knew the Southern Hemisphere didn’t see much flu, so we were preparing for the twindemic and hoping for very little flu,” Martinello said. “We’re still really amazed to see there is essentially none. And that’s while testing over a thousand patients for flu. It’s not like we’re not looking for it because we’re only concerned about COVID.”

Martinello said that as of a week ago — the last week for which he’d seen data — Yale New Haven Health, which includes Lawrence + Memorial Hospital in New London and Westerly Hospital, had not treated a single case of flu.

“If we had had a case last week, I would have heard about it,” he said.

In Connecticut, 58 positive influenza tests had been reported this season to the state Department of Public Health as of the week that ended Feb. 13. According to the department, which closely tracks flu every year, influenza-associated hospitalizations this season numbered 13, and one death had been linked to the flu. During the previous flu season, when it stopped reporting cases in April 2020, the department counted 3,013 cases and 79 deaths, including one of a child.

The U.S. Centers for Disease Control and Prevention reports that the diminution of the flu is occurring across the country.

The centers’ Influenza Hospitalization Surveillance Network, which gathers data on laboratory-confirmed, influenza-related hospitalizations in 14 states representing about 9% of the U.S. population, counted 183 hospitalizations between Oct. 1, 2020, and Feb. 20, 2021, a rate of 0.6 hospitalizations per 100,000 population. The rate is much lower than the average for this point in the season and lower than the rate for any season since routine data collection began in 2005. During the 2011-12 season, a low point in flu severity, the rate was 2.2 times higher than it is at this time, according to the CDC.

Only one influenza-associated pediatric death has been reported to the CDC during the current flu season.

Keeping the flu at bay

Respiratory syncytial virus, or RSV, another seasonal virus that afflicts most children by the age of 2 and can have serious consequences for those with certain conditions, also has gone virtually undetected this season, according to Martinello. The same virus can cause pneumonia in older adults and annually causes hundreds of deaths in the U.S.

While a phenomenon known as “viral interference” — the ability for one viral infection to briefly provide a person with protection against another viral infection — may be at play to some extent with COVID-19 and the flu, Martinello said it’s largely the public’s adherence to COVID-19 protocols that has kept the flu at bay.

“Health measures put in place for COVID have been very effective in protecting us from other maladies,” he said. “While COVID may be widespread, it’s still a very small percentage of the population that’s been infected with it. ... The bulk of what we’re seeing (regarding flu) is the result of people staying home, mask-wearing, distancing and washing their hands.”

It’s also likely that more people than ever before have gotten flu shots this season, though data that could confirm or refute that assumption are not yet available. In a September survey, the CDC found that 59% of U.S. adults had been or intended to be vaccinated for the flu this season, compared with 52% who reported getting a flu shot the previous season.

Martinello cautioned that the current season isn’t over yet and that cases of the flu could yet emerge.

“Usually, we see hundreds of cases going into the new year, and in mid to late January we see an upswing,” he said. “Sometimes it peaks in March but most often February, and then tails off in April and May. ... We’re not entirely out of the woods yet.”

Unfortunately, the severity of any flu season has no effect on the following flu season. Mild years have been followed by severe years and vice versa.

"The flu will come back," Martinello said.

b.hallenbeck@theday.com