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The Co-Infection Convergence : SARS-CoV-2 and Influenza
By Kelly M . Pyrek
We must use the interventions that evidence supports impacting spread of SARS-
CoV-2 ( which also should impact spread of influenza and other respiratory viruses ) to reduce morbidity and mortality , but we also must continue to investigate the nuances of
COVID-19 .”
— Thomas Talbot , MD ,
MPH
Comparisons between SARS-CoV-2 and the influenza virus were drawn in the early days of the outbreak and subsequent pandemic , and with flu season upon us , U . S . public health experts are pondering what the winter season in the Northern Hemisphere will bring and if healthcare institutions are prepared .
That SARS-CoV-2 continues to evolve at the same time that the influenza viruses continue to mutate is a concern , and as the American Society for Microbiology ( 2020 ) observes , “ SARS-CoV-2 is a novel virus , which means that we had no available treatments or immunity to the pathogen when it emerged in late 2019 . Because of this , the virus was able to spread , unrestrained from host to host , and it didn ’ t take long for the SARS-CoV-2 outbreak to become a pandemic . Even though the flu is not a new pathogen , influenza virus is constantly evolving and experiencing varying levels of antigenic drift ( and shift ) that can make it less recognizable to our immune systems . This has made the development of a universally effective flu vaccine particularly challenging and explains the constant underlying threat that new zoonotic influenza strains might emerge and become pandemic . All cases of pandemic flu in the U . S . have been caused by zoonotic subtypes of influenza A , including the Flu of 1918 ( H1N1 ), the 2005 Avian flu ( H5N1 ) and the 2009 Swine flu ( H1N1 ).”
Maintaining evidence-based infection prevention and control measures as well as continuing to strive for additional knowledge about SARS-CoV-2 infectivity and transmission is the approach that is being embraced by most experts , including Thomas Talbot , MD , MPH , chief epidemiologist at Vanderbilt University Medical Center . “ I think we must continue to do both and that these functions are not mutually exclusive ," Talbot emphasizes . " We must use the interventions that evidence supports impacting spread of SARS-CoV-2 ( which also should impact spread of influenza and other respiratory viruses ) to reduce morbidity and mortality , but we also must continue to investigate the nuances of COVID-19 . Just as we still learn aspects about influenza today , we cannot stop working to better understand COVID-19 .”
While both viruses cause respiratory disease , there are important differences between the pathogens and how they spread , with significant implications for how they can be controlled . Both viruses are transmitted by contact , droplets and fomites , but the similarities end there ; according to public health experts , the speed of transmission is an important point of difference between the two viruses . Influenza has a shorter median incubation period ( the time from infection to appearance of symptoms ) and a shorter serial interval ( the time between successive cases ) than COVID-19 virus . The serial interval for COVID-19 virus is estimated to be five to six days , while for influenza virus , the serial interval is about three days , meaning that influenza can spread faster than COVID-19 . Further , transmission in the first three to five days of illness , or potentially pre-symptomatic transmission – transmission of the virus before the appearance of symptoms – is a major driver of transmission for influenza .
Scientists and public health experts are learning that there are individuals who can shed COVID-19 virus 24 to 48 hours prior to symptom onset , but , as the World Health Organization reports , “ The reproductive number – the number of secondary infections generated from one infected individual – is understood to be between 2 and 2.5 for COVID-19 virus , higher than for influenza . However , estimates for both COVID-19 and influenza viruses are very context and time-specific , making direct comparisons more difficult .”
Regarding the clinical presentation of influenza versus COVID-19 , the NIH ( 2020 ) explains , “ The signs and symptoms of uncomplicated , clinically mild influenza overlap with those of mild COVID-19 . Ageusia and anosmia can occur with both diseases , but these symptoms are more common with COVID-19 than with influenza . Fever is not always present in patients with either disease , particularly in patients who are immunosuppressed or elderly . Complications of influenza and COVID-19 can be similar , but the onset of influenza complications and severe disease typically occurs within a week of illness onset whereas the onset of severe COVID-19 usually occurs in the second week of illness . Because of the overlap in signs and symptoms , when SARS-CoV-2 and influenza viruses are cocirculating , diagnostic