What happened to the flu season in 2020–2021?

A: In the Northern Hemisphere, influenza season typically starts around October and peaks between December and February. We often see cases—and in some seasons a second or even a third peak—as late as April or May, as it happened in 2019−2020 (the last pre-COVID-19 flu season). Influenza tends to spike in the fall and winter because the virus survives better in colder, dry temperatures. In a typical flu season, we see a 10%–20% positive rate among patients tested for a flu-like illness.

Flu activity in the U.S. during the 2019–2020 season began to increase in November and was consistently high through January and February 2020. However, the COVID-19 pandemic dramatically decreased the number of influenza cases in the next flu season, 2020–2021, making the virus almost disappear.

According to the Centers for Disease Control and Prevention (CDC), there have been about 1,400 positive flu cases in 2020−2021, while the previous year included more than 130,000 positive test results. The hospitalization rate in the 202−2021 flu season was just 0.7 per 100,000 people, the lowest since the CDC started to collect data in 2005. COVID-19 preventive measures such as using face masks, social distancing, staying home, school closures, improved indoor ventilation, and hand washing all were effective countermeasures that likely contributed not only to successes in the fight against COVID-19 but also to the low incidence of flu and lower hospitalizations and deaths due to infection with this virus. Another contributing factor was the high rate of influenza vaccination. A record number of flu vaccine doses (more than 190 million) were distributed in the U.S. during 2020−2021.

After this almost nonexistent flu season, a more brutal season was projected last fall for 2021−2022, with the specter of a “twindemic”. Clinical laboratories and the major diagnostic manufacturers have worked hard to prepare for this scenario. Both high-throughput and rapid SARS-CoV-2 plus influenza A/B combination molecular tests became available shortly before the new season started.

Where are we headed this year?

Flu seasons are notoriously difficult to predict due to the virus’s rapidly changing genetic makeup, its biological behavior, and the multiple factors affecting its spread. After the quiet summer for COVID-19 in 2021, and with high full vaccination rates, some expected a repeat of the post-1918 flu pandemic merriment, with a new “roaring 20s” in the fall: Without masks and social distancing, extensive travel and social activities would resume—all contributing factors to a potentially heavy flu season.

Then came the SARS-CoV-2 delta variant, and last December, the super-contagious omicron variant with very high infection rates but relatively low mortality. By mid-January, this year we witnessed record numbers of SARS-CoV-2 infections, record testing volumes, and booster shot requirements for healthcare workers and others.

Meanwhile, influenza cases had emerged last November and reached a mild peak in December. This was considerably greater than the same time period a year earlier, but far below the numbers of previous regular flu seasons. Sporadic influenza activity continued across the country in January and early February. The majority of influenza viruses detected have been A (H3N2). According to the CDC, of the 6,774 influenza positives reported so far this season by public health laboratories in which SARS-CoV-2 testing was also performed, 402 (5.9%) were also positive for SARS-CoV-2. Cases of SARS-CoV-2 omicron have shown a precipitous decline and there is no immediate threat of a new variant on the horizon.

The positive developments with COVID-19 will likely lead to an easing of preventive measures and mandates, potentially resulting in a spring spike of flu. Fortunately, clinical laboratories are well prepared to meet the challenges ahead.

Gyorgy Abel, MD, PhD, is medical director of molecular diagnostics at Lahey Hospital and Medical Center in Burlington, Massachusetts. +Email: [email protected]