Low flu cases spotlight COVID-19 transmissibility
As COVID-19 raged last year, the seasonal flu all but vanished, according to data from the U.S. Centers for Disease Control and Prevention.
During the 2019 flu season from Sept. 29 to Dec. 28, the CDC reported more than 65,000 cases of influenza nationwide. During the same period last year, the agency reported 1,016 cases.
Health experts said that high vaccination rates against the flu – combined with social distancing, mask-wearing and hand-washing employed to stop the spread of the coronavirus – played a huge role in preventing influenza transmission.
The drop occurred despite a sixfold increase in testing at public health labs, most of which checked for influenza A and B along with the coronavirus.
Clinical lab testing was slightly lower during the last quarter of 2020 as physicians ordered fewer flu tests because less of the illness was circulating.
“The public health labs test for more surveillance purposes rather than patient care reasons and are therefore a better measure of influenza burden each season than clinical labs,” CDC spokesperson Kate Grusich told USA TODAY.
Though many experts are relieved to see public health measures working against flu spread, they said the numbers speak volumes about the transmissibility of COVID-19.
“It says that it’s more contagious and that it’s less forgiving of any lapses of these types of prevention measures,” said David Hooper, chief of the infection control unit at Massachusetts General Hospital.
Hooper said one reason the coronavirus is more transmissible is because people can shed the coronavirus days before exhibiting any symptoms, if they develop symptoms at all.
A model developed by CDC researchers and published Thursday in JAMA Network Open found that people who don’t show symptoms may be responsible for 59% of COVID-19 transmissions, comprising 35% who are pre-symptomatic and 24% who never develop symptoms.
People generally don’t shed flu virus for more than a day before symptoms appear, Hooper said.
Susan Rehm, vice chair at the Cleve
land Clinic’s department of infectious diseases, said another reason flu incidences are low is because most people have some innate immunity from prior vaccinations and infections.
“COVID is a novel infection caused by the SARS coronavirus, and no one has any innate immunity to it,” she said. “So the population is probably overall more susceptible to it than maybe to influenza.”
It helps that Americans vaccinated against the flu at record numbers last year compared with previous seasons, Rehm said.
As of Dec. 25, more than 192 million doses of flu vaccine had been distributed, “which is the highest number of doses distributed in the U.S. in a single flu season,” Grusich said. Flu vaccine manufacturers project they will provide as many as 194 to 198 million doses to the U.S. market by the end of the season, which could last as late as May, according to the CDC.
Rehm said Americans were especially motivated to get a flu vaccine last year as health experts warned hospitals could be overwhelmed by flu and
COVID-19 patients in a “twindemic” scenario.
“A lot of people in the past haven’t felt that flu was very severe and thus haven’t necessarily felt so motivated to get vaccinated,” she said. “Certainly, COVID has taught us that respiratory illnesses can be extremely severe.”
Though the news about flu is good, Rehm cautioned the season is not over.
“Just because it’s been low so far doesn’t prove that it’s going to be low going forward, and getting vaccinated is the best thing you can do to prevent influenza,” she said. “It’s not too late to get vaccinated for influenza.”
“Just because it’s been low so far doesn’t prove that it’s going to be low going forward, and getting vaccinated is the best thing you can do to prevent influenza. It’s not too late to get vaccinated for influenza.” Susan Rehm Vice chair at the Cleveland Clinic’s department of infectious diseases