How COVID-19 Measures Nearly Erased the Flu in 2020

When SARS-CoV-2, the virus that causes COVID-19, began spreading widely in the community in mid- to late February 2020, something else happened too: influenza began to vanish. CDC set out to learn how much of that was real, and how much was simply fewer people getting tested.

The United States

CDC analyzed data from about 300 clinical laboratories in all 50 states, DC, Puerto Rico and Guam. The flu season had been strong: more than 20% of respiratory specimens tested positive from mid-December 2019 into early March, peaking at 30.25%. Then the national emergency was declared on March 1, 2020, and flu collapsed.

• From its peak, positivity fell 14.90% by week 9 — but 89.77% in weeks 10–13, after the emergency declaration.
• By the week of March 22, with testing still very high, just 2.3% of specimens were positive; from the week of April 5 it stayed below 1%.
• Summer flu (May 17 – August 8) hit a historic low: a weekly median of 0.20% positive in 2020, against 2.35% in 2019, 1.04% in 2018 and 2.36% in 2017.

Flu tests and percent positive by week in the United States, 2019–20, with three earlier seasons for comparison. Image from CDC’s report.

The Southern Hemisphere

June to August is normally flu season south of the equator. CDC looked at three countries that serve as strong sentinel sites — Australia, Chile and South Africa — for April–July 2020:

In the same months of 2017–2019, 24,512 of 178,690 specimens (13.7%) had tested positive.

Each country had imposed strict measures: Australia a 14-day hotel quarantine for returning travelers (March 29), regional lockdowns and gathering bans; Chile a state of emergency from March 18, curfews, a nationwide lockdown and mandatory masks; South Africa a total lockdown from April 9. The United States saw school closures, bans on mass gatherings, stay-at-home orders, masks and distancing.

Real, not just less testing

At first the drop was blamed on less testing, since people with respiratory symptoms were steered toward COVID-19 tests. But when health officials pushed to test for flu again, they tested enough samples and still found little or none. Australia loosened its testing criteria and tested far more specimens than usual — and still found few cases at the usual peak.

The same decline appeared in other Northern Hemisphere countries and the tropics. Flu, like SARS-CoV-2, spreads mainly by droplets, and it is less transmissible (R0 of 1.28, against 2–3.5 for SARS-CoV-2) — which likely made it easier to interrupt.

What it means

• Measures used against COVID-19, plus flu vaccination, could sharply reduce flu in the 2020–21 Northern Hemisphere season, and some might help against flu in future seasons, especially for people at highest risk.
• It is hard to separate the effect of each measure: children drive flu spread, but school closures alone have unclear effect because adults have other exposures. Evidence supports masks worn by infected people to protect others, and growing evidence supports them for protecting the wearer. How acceptable a measure is likely falls as it gets stricter.
• Because the pandemic and its measures were uncertain, CDC urged planning for a normal flu season. Flu vaccination for everyone 6 months and older remained the best protection — especially with SARS-CoV-2 and flu able to circulate together.

Limits: this kind of comparison cannot prove cause; less travel and more vaccination may also have helped; SARS-CoV-2 might have crowded out flu in the airways, though U.S. flu was already falling before COVID-19 spread widely; and the season might simply have been ending — though the drop after March 1 was unusually steep.

Sources

Based on 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 Vol. 69, No. 37, CDC; a work of the United States government in the public domain.