There are encouraging signs that masking and other measures taken to prevent COVID-19 are diminishing seasonal influenza globally. Flu virus circulation declined when COVID-19 measures were taken in the Northern Hemisphere, with the same epidemiology observed as the flu season began later below the equator, the Centers for Disease Control and Prevention (CDC) reported.1

“Following widespread adoption of community mitigation measures to reduce transmission of SARS-CoV-2, the percentage of U.S. respiratory specimens submitted for influenza testing that tested positive decreased from > 20% to 2.3% and has remained at historically low interseasonal levels (0.2% vs. 1-2%),” the CDC noted. “Data from Southern Hemisphere countries also indicate little influenza activity.”

Although there are caveats and cautions, the CDC is leaning into the findings, and even suggested they may carry implications for future flu seasons.

“Although causality cannot be inferred from these ecological comparisons, the consistent trends over time and place are compelling and biologically plausible,” the CDC said. “Like SARS-CoV-2, influenza viruses are spread primarily by droplet transmission; the lower transmissibility of seasonal influenza virus (R0 = 1.28) compared with that of SARS-CoV-2 (R0 = 2-3.5) likely contributed to a more substantial interruption in influenza transmission.”

The formula for continuing this trend as the 2020-21 flu season begins in the United States is high immunization levels for influenza and the measures that have become standard for COVID-19: masking, social distancing, and hand hygiene.

“I have had some communications with colleagues in Australia and they did indeed have a very mild influenza season,” says William Schaffner, MD, professor of preventive medicine at Vanderbilt University. “They attribute it to two things. The first is that they used influenza vaccine more than they ever have before — they really promoted influenza immunization. The other is, of course, the COVID precautions. They were far from perfect, but they have the sense that the country was reasonably compliant — more so than we are here in the United States.”

There may be the rub, as mask use became politicized and undermined early in the pandemic in the United States. “There are lots of people who have COVID fatigue and wish to go back to the old normal. There is a thrust of that certainly in our part of the country,” Schaffner says. “When you get out of the cities and go into the country, there is not only an aversion to wearing masks, but real hostility. These COVID-19 interventions have been so politicized that is has been a huge impediment to solid, sensible public health.”

Questionable compliance makes the flu vaccine more important. It appears there is a good match between the shot and circulating virus, says Schaffner, who leads the annual seasonal flu press conference at the National Foundation for Infectious Diseases. “One message strongly reinforced every year is that flu is infamously unpredictable.”

“Any one year it could be mild, moderate, or severe,” adds Richard Wenzel, MD, MSc, emeritus chairman and professor of internal medicine at Virginia Commonwealth University. “In the last decade — where they have counted deaths the same way — it was as low as 12,000 to a high of about 86,000 deaths.”

Based on the forecast that the circulating flu will leave a smaller footprint, COVID-19 precautions could halve the aforementioned 12,000 deaths of a mild season, he says.

“I think it will cut down flu as much as 50% or more if we continue to use the masks. Hopefully, we will,” Wenzel says. “Even when the flu vaccine begins to roll out, people should continue to wear masks because we won’t have any kind of herd immunity [for COVID-19] until the end of spring next year.”

The findings raise the question of whether masking will be encouraged in post-pandemic flu seasons. “There is evidence to support the use of face masks by infected persons to reduce transmission of viral respiratory illnesses to others and growing evidence to support their use (in the healthcare setting, in households, and in the community) to protect the healthy wearer from acquiring infection,” the CDC noted. “Data from the current pandemic might help answer critical questions about the effect of community mitigation measures on transmission of influenza or other respiratory diseases.”

A tool to make that critical distinction is a new Food and Drug Administration-approved multiplex diagnostic assay for detection of both SARS-CoV-2 and influenza virus that could improve future surveillance efforts.2

“Assessing acceptability of effective measures would be critical, because acceptability is likely to be inversely correlated with the stringency of the measure,” the CDC cautioned.

REFERENCES

  1. Olsen SJ, Azziz-Baumgartner E, Budd AP, et al. Decreased influenza activity during the COVID-19 pandemic — United States, Australia, Chile, and South Africa, 2020. MMWR Morb Mortal Wkly Rep 2020;69:1305-1309.
  2. Centers for Disease Control and Prevention. CDC’s diagnostic multiplex assay for flu and COVID-19 at public health laboratories and supplies. Sept. 3, 2020. https://www.cdc.gov/coronavirus/2019-ncov/lab/multiplex.html