As Olsen , et al . ( 2020 ) explain , “ In countries or jurisdictions where extensive community mitigation measures are maintained ( e . g ., face masks , social distancing , school closures , and teleworking ), those locations might have little influenza circulation during the upcoming 2020 – 21 Northern
Hemisphere influenza season .” remained at historically low inter-seasonal levels ( 0.2 percent versus 1-2 percent ).” They emphasize that , “ Interventions aimed against SARS-CoV-2 transmission , plus influenza 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 .”
The authors explain , “ These changes were attributed to both artifactual changes related to declines in routine health seeking for respiratory illness as well as real changes in influenza virus circulation because of widespread implementation of measures to mitigate transmission of SARS-CoV-2 . Data from clinical laboratories in the United States indicated a 61 percent decrease in the number of specimens submitted ( from a median of 49,696 per week during Sept . 29 , 2019 to Feb . 29 , 2020 , to 19,537 during March 1 to May 16 , 2020 ) and a 98 percent decrease in influenza activity as measured by percentage of submitted specimens testing positive ( from a median of 19.34 percent to 0.33 percent ). Interseasonal ( i . e ., summer ) circulation of influenza in the United States ( May 17 to Aug . 8 , 2020 ) is currently at historical lows ( median = 0.20 percent tests positive in 2020 versus 2.35 percent in 2019 , 1.04 percent in 2018 , and 2.36 percent in 2017 ).”
As Olsen , et al . ( 2020 ) explain , “ In countries or jurisdictions where extensive community mitigation measures are maintained ( e . g ., face masks , social distancing , school closures , and teleworking ), those locations might have little influenza circulation during the upcoming 2020-21 Northern Hemisphere influenza season . The use of community mitigation measures for the COVID-19 pandemic , plus influenza vaccination , are likely to be effective in reducing the incidence and impact of influenza , and some of these mitigation measures could have a role in preventing influenza in future seasons . However , given the novelty of the COVID-19 pandemic and the uncertainty of continued community mitigation measures , it is important to plan for seasonal influenza circulation in the United States this fall and winter . Influenza vaccination of all persons aged ≥6 months remains the best method for influenza prevention and is especially important this season when SARS-CoV-2 and influenza virus might cocirculate .”
According to the researchers , in the U . S ., influenza virus circulation declined sharply within two weeks of the COVID-19 emergency declaration and widespread implementation of community mitigation measures , including school closures , social distancing , and mask wearing , although the exact timing varied by location . They add that the decline in influenza virus circulation observed in the U . S . also occurred in other Northern Hemisphere countries and the tropics , and the Southern Hemisphere temperate climates have had virtually no influenza circulation .
They note , “ Although causality cannot be inferred from these ecological comparisons , the consistent trends over time and place are compelling and biologically plausible . 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 . These findings suggest that certain community mitigation measures might be useful adjuncts to influenza vaccination during influenza seasons , particularly for populations at highest risk for developing severe disease or complications .”
They continue , “ 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 United States might remain low and the season might be blunted or delayed . 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 . However , in light of the novelty of the COVID-19 pandemic and the uncertainty of continued community mitigation measures , it is important to plan for seasonal influenza circulation this fall and winter .”
In an attempt to explain the uncharacteristically low reported incidence of seasonal influenza in the southern hemisphere so far this year , the American Society for Microbiology ( 2020 ) says , “ It may be that the case load simply appears lower because of insufficient testing and reporting , or that the social distancing measures put in place to help stop the transmission of SARS-CoV-2 have reduced the transmission of influenza virus as well . Whether either virus causes viral interference ( competitively suppresses replication of the other virus ) or modulates disease severity is of great interest . Because SARS-CoV-2 and influenza virus both infect cells of the respiratory tract , they might have to compete for resources ( including cells to infect ) during co-infection . The host cell receptors are unique for each of these viruses . Sialic acid is more prevalent than ACE2 , but the binding affinity of S protein to ACE2 is remarkably strong . It is possible , but remains uncertain , whether either of these factors contributes a competitive advantage . The host immune response presents another variable worth considering . Does a host ’ s immune response to one virus make it more difficult for the other to cause infection ? Or is an already immunocompromised host left more vulnerable to secondary infection ?