Covid-19 lockdowns, masks and social distancing likely kept flu at 'historical lows' in US and globally: CDC

According to experts, some of these mitigation steps could be implemented in the future during flu epidemics to reduce transmission, particularly in populations at the highest risk for developing severe disease

                            Covid-19 lockdowns, masks and social distancing likely kept flu at 'historical lows' in US and globally: CDC
(Getty Images)

Community mitigation measures to check the spread of Covid-19, such as stay-at-home orders, face masks, social distancing, teleworking, and school closures may have kept cases of the flu at “historical lows” in the US and other parts of the world, according to the Centers for Disease Control and Prevention (CDC).

Overall, data from clinical laboratories in the US indicate a 61% decrease in the number of respiratory specimens that were submitted for the flu test -- from a median of 49,696 per week during September 29, 2019-February 29, 2020, to 19,537 during March 1-May 16, 2020. During this period, there was a 98% decrease in flu activity as measured by the percentage of submitted specimens testing positive, from a median of 19.34% to 0.33%. Interseasonal, that is, summer circulation of flu in the US (May 17-August 8, 2020) is currently extremely low, with just 0.20% of samples testing positive in 2020 versus 2.35% in 2019, 1.04% in 2018, and 2.36% in 2017, reveals analysis.

“Following widespread adoption of community mitigation measures to reduce transmission of SARS-CoV-2, the virus that causes Covid-19, the percentage of US respiratory specimens submitted for influenza testing that tested positive decreased from more than 20% to 2.3% and has remained at historically low interseasonal levels. Data from Southern Hemisphere countries also indicate little influenza activity,” write authors. 

Data from approximately 300 US clinical laboratories located across all 50 states, Puerto Rico, Guam, and the District of Columbia were used for this analysis. To assess influenza virus activity in the Southern Hemisphere, influenza laboratory data from clinical and surveillance platforms reported from Australia, Chile, and South Africa to the World Health Organization’s FluNet platform were analyzed. 

The research team found that in the US, flu activity (measured by the percentage of respiratory specimens submitted for flu testing that yielded positive results) began to increase in early November 2019, and more than 20% of specimens were positive during December 15, 2019-March 7, 2020, after which activity declined sharply. By the week of March 22, 2020, when the number of samples tested remained very high, percent positivity dropped to 2.3%, and since the week of April 5, 2020, has remained less than 1%. 

Following the widespread adoption of community mitigation measures to reduce transmission of coronavirus, the percentage of US samples submitted for flu testing that tested positive decreased from over 20% to 2.3% (Getty Images)

In the Southern Hemisphere countries of Australia, Chile, and South Africa, only 33 influenza-positive test results were detected among 60,031 specimens tested in Australia, 12 among 21,178 specimens tested in Chile, and six among 2,098 specimens tested in South Africa, for a total of 51 influenza-positive specimens (0.06%) among 83,307 tested in these three countries during April-July 2020. In contrast, during April-July in 2017-2019, 24,512 specimens tested positive for influenza (13.7%) among 178,690 tested in these three countries. 

“In the US, influenza virus circulation declined sharply within 2 weeks of the Covid-19 emergency declaration and widespread implementation of community mitigation measures, although the exact timing varied by location. The decline in influenza virus circulation observed in the US also occurred in other Northern Hemisphere countries and the tropics, and the Southern Hemisphere temperate climates have had virtually no influenza circulation. Although causality cannot be inferred from these ecological comparisons, the consistent trends over time and place are compelling and biologically plausible,” explain authors. They add, “Like SARS-CoV-2, influenza viruses are spread primarily by droplet transmission; the lower transmissibility of seasonal influenza virus compared with that of SARS-CoV-2 likely contributed to a more substantial interruption in influenza transmission.”

The team says that initially, declines in flu virus activity were attributed to decreased testing because persons with respiratory symptoms were often preferentially referred for Covid-19 assessment and testing. However, renewed efforts by public health officials and clinicians to test samples for influenza resulted in adequate numbers tested and detection of little to no influenza virus, they add. 

The researchers recommend that interventions aimed against Covid-19 transmission along with flu vaccination could substantially reduce influenza incidence and impact in the 2020-21 Northern Hemisphere season. Some mitigation measures might have a role in reducing transmission in future influenza seasons, they suggest. However, CDC emphasizes that flu vaccination for all persons aged 6 months and older “remains the best method” for flu prevention and is “especially important this season” when coronavirus and influenza virus might cocirculate.

“The global decline in influenza virus circulation appears to be real and concurrent with the Covid-19 pandemic and its associated community mitigation measures. Influenza virus circulation continues to be monitored to determine if the low activity levels persist after community mitigation measures are eased. If extensive community mitigation measures continue throughout the fall, influenza activity in the US might remain low and the season might be blunted or delayed,” recommend authors. They add, “In the future, some of these community mitigation measures could be implemented during influenza epidemics to reduce transmission, particularly in populations at highest risk for developing severe disease or complications.”

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