FIGURE 4 : Average ( 7-day moving ) number of tests performed for SARS- CoV-2 and influenza from December 2018 through June 2022 .
predictably decreased during summer months while SARS-CoV-2 testing continued throughout surges . The average number of tests increased to 543 from January 2022 through March 2022 during the 2021 / 2022 season . This increase in testing during the omicron surge is attributed to increased testing for SARS-CoV-2 and increased use of multiplexed molecular assays that test for both viruses using a single patient sample . Many of these tests were performed in outpatient settings ( 63 %). There were strong negative correlations between SARS-CoV-2 and influenza test positivity during both omicron surges ( December 2021-March 2022 , -0.23 , p = 0.024 ; April 2022-June 2022 , -0.61 , p < 0.001 ).
Of 82,751 individuals who tested positive for SARS-CoV-2 , 17,944 also had an influenza result within 14 days on either side of the positive COVID result . Less than 1 % ( N = 76 or 0.4 %) were positive for influenza . Most of these tests were performed on the same day or within one day of each other , representing coinfections .
DISCUSSION The interplay between SARS-CoV-2 and
influenza during the study period was dynamic . Influenza positivity rates approached 30 % in early 2020 and then fell sharply , never returning to more than 10 % to date and remaining under 5 % for most of the pandemic . This rate is below the 7.2 % from the pre-COVID era . The initial decrease in influenza positivity is easier to understand than later patterns . Mitigation measures were put into effect in late February and early March 2020 . These measures included stay-at-home orders , facility closures , mask wearing mandates , social distancing , and bans on mass gatherings . Some or all of these interventions could decrease transmission of influenza ; indeed , similar decreases in positivity rates were reported around the world . 4 The negative correlation between influenza and SARS-CoV-2 test positivity is also not unique to WV and has been seen in other countries . 5 This decrease in test positivity is not universal , however , as some parts of the world experienced a general decrease in positive tests but a consistent influenza positivity rate . 6
The first two traditional flu seasons of the pandemic presented contrasting events in
WV . Winter 2020 was associated with a SARS-CoV-2 surge with virtually no influenza season . Winter 2021 was associated with another SARS-CoV-2 surge , but this time there were two upward trends in positivity rates for influenza , one starting in late December and another in the spring . The difference may be attributed to mitigation methods in winter 2020 that were not present in winter 2021 , such as the state-wide indoor masking mandate that was lifted on June 20 , 2021 . 7 Another hypothesis is that the viral interactions that occur between early variants of SARS- CoV-2 and influenza are different than those occurring with later variants ( e . g ., omicron ) that are phylogenetically distant from earlier variants . 8 , 9
Viral interactions among respiratory viruses are well described . While the specific interactions between viruses vary , viral interference is thought to be caused by a temporary increase in innate immunity to the second virus due to an interferon response caused by the initial infection attributed to the first virus . 10 Research in this area has been variable . Pig models reveal that influenza could inhibit corona-
26 • www . wvsma . org