As COVID-19 cases continued to rise this past summer, Eili Klein worried that his children might need to stay home from school, but not about the severity of the upcoming flu season.

The Johns Hopkins epidemiologist already knew that flu transmission would be reduced because people were following the public health safety precautions recommended to prevent acquiring the novel coronavirus.

With people still wearing masks, washing their hands frequently and physically distancing, he expected these measures to temper the flu this year. And, with most public schools — major transmission routes for flu — likely remaining closed, in addition to restaurant and business closures, there were even fewer opportunities for flu to spread.

Klein was correct. The flu season generally peaks between December and February each year, bringing up to 45 million illnesses, 810,000 hospitalizations and 61,000 deaths, according to the Centers for Disease Control and Prevention (CDC). This year, however, as of Feb. 5, there have been only 1,455 cases in the U.S.

“Though caused by a different virus from the one that causes COVID-19, the flu is also a respiratory viral disease, so everything we are doing to slow transmission of COVID-19 should also reduce transmission of flu,” says Klein, an associate professor of emergency medicine at the Johns Hopkins University School of Medicine. (Read about how to protect yourself from the flu and COVID-19.)

By Jan. 31, there had been only six cases of flu diagnosed this season at Johns Hopkins hospitals, including The Johns Hopkins Hospital, Johns Hopkins Bayview Medical Center, Howard County General Hospital, Sibley Memorial Hospital, Suburban Hospital and Johns Hopkins All Children’s Hospital. Those hospitals saw 4,805 cases in the 2019–20 season, and 2,846 in the 2018–19 season.

(Because the flu season typically runs from Oct. 15 to May 15, there’s still a chance that flu cases will pick up in the next few months. Also, one strain of flu — influenza B — tends to circulate later in the season.)

Vaccinations have also helped prevent infection. More Americans received the flu vaccine this season than in the previous four flu seasons, according to the CDC. By the end of January 2021, 193.2 million people had been vaccinated, compared with 173.3 million at that time last year.

At Johns Hopkins Medicine, which includes the Johns Hopkins University School of Medicine and the Johns Hopkins Health System, the flu vaccine is mandatory for all personnel, unless they have approved medical or religious exemptions.

Cases of other respiratory viruses besides influenza are down as well; the health system diagnosed only six positive cases of respiratory syncytial virus (RSV) by the end of January. By this time in the previous two years, Johns Hopkins clinicians had already seen hundreds of RSV cases.

This good news does, however, have a flip side. Klein believes that if this flu season remains less severe than usual, next year’s season could be worse.

“Because of the current restrictions and precautions everyone is taking this season, far fewer people will be infected or exposed to the flu virus, and therefore won’t become immune to certain strains of the virus,” he says. “So, the number of people who may have more severe infections next year is likely to be greater because immunity will be lower.”

The fact that flu cases have been reduced but COVID-19 continues to spread, Klein says, leads to several conclusions about how the viruses behave. First, the virus that causes COVID-19 is very transmissible, given that it continues to spread — even with various public health measures in place. This also points to how easily respiratory viruses can spread through asymptomatic carriers, Klein says.

A reduction in flu cases also shows that public health measures are effective. Klein says even he’s surprised at the number of COVID-19 cases being seen in Maryland as of mid-January.

He adds that the willingness of Marylanders to follow physical distancing guidelines, as well as the closing of restaurants and other businesses, have significantly helped keep the winter surge from overwhelming the health system.

“Overall, the cases and occupancy numbers have been on the optimistic side of the model projections,” Klein says. “Far fewer people died than would have if we had not followed those guidelines.”