After an extremely mild Southern Hemisphere influenza season, insurers are now turning their attention to the upcoming Northern Hemisphere flu season.
Several papers have explored the impact of COVID-19 on influenza or other viral infections. Researchers are watching to see if Northern Hemisphere locations will experience a dual influenza and COVID-19 season – a so-called “twindemic” – or if influenza will remain at historically low levels while COVID-19 is predominant.
Slow Start to Flu Season in the Northern Hemisphere
Winter weather generally corresponds with an uptick in the number of influenza cases. This year, while infections of SARS-CoV-2 (the virus that causes COVID-19) have increased going into the Northern Hemisphere fall, laboratory-confirmed influenza cases have been surprisingly absent. In the U.S. (see Figure 1), for example, even the slowest start from the previous five years (2015-16 season) had 3.7 to 7.7 times the number of cases as the current season at this time (weeks 40-45), based on reporting from clinical and public health laboratories.(1) Europe, through week 44, has seen a 97% reduction in detections compared to the same time period in 2019. (2) Canada has reported just 19 influenza detections between weeks 35-46 compared to an average of 1,115 over the last six seasons. This decrease in infections in Canada is despite a 2.5 times increase in the average number of tests, leading to 0.4% of tests positive for influenza in week 46 compared to 6.1% during the past six seasons. (3)
Figure 1: U.S. Positive Influenza Tests
In the U.S., only a fraction of 1% of reported influenza tests are positive. By contrast, 11.9% of U.S. SARS-CoV-2 tests were positive in week 46. (4)
While it is still too early to determine if the influenza season will peak later than usual or pass with a historically low number of cases, the spotlight is currently on SARS-CoV-2.
COVID-19 vs. Influenza
Data from mid-2020 indicated a mild influenza season during winter in the Southern Hemisphere. The question surfaced: To what degree was this due to community COVID-19 mitigation measures such as lockdown and social distancing, reduction in travel, school closures, increased influenza vaccine use, hand hygiene, and mask wearing – all of which are also known to prevent or reduce influenza transmission? (5)
It is known that some viruses (including influenza) cause interference and trigger an immune response, helping prevent further infections by other viruses.(6) Researchers are investigating how this phenomenon might play out with influenza and SARS-CoV-2.
Coinfection with both viruses is possible, and patients coinfected with both influenza and SARS-CoV-2 have worse outcomes. (7; 8) However, there appears to be some interference and competition between the viruses. Some preliminary research indicates the interference effect may also work for the influenza vaccine, and it could play a favorable role in mitigating COVID-19 severity. Early studies in a few countries appear to support this theory.(8) (9) (10) (11) (12) However, at least one pre-COVID-19 study showed that while influenza vaccination reduced rates of some non-influenza respiratory viruses, odds of coronavirus infections increased (13).
Heading into 2021, insurers must keep an eye on early indicators of morbidity and mortality related to case counts for both influenza and COVID-19, including percentages of emergency department and outpatient visits, hospitalization rates due to each virus, and actual versus expected death counts. While the SARS-CoV-2 pandemic remains a historic global challenge, the possibility of a milder-than-normal influenza season may provide some much-needed relief for both society and insurers.
- CDC. National Summary, week 45 ending Nov 7, 2020. Influenza Positive Tests Reported to CDC by Public Health and Clinical Laboratories. [Online] FluView Interactive, Nov 7, 2020. [Cited: November 19, 2020.] https://gis.cdc.gov/grasp/fluview/fluportaldashboard.html.
- European Centre for Disease Prevention and Control. Influenza virus characterisation - Summary Europe, October 2020. ECDC Surveillance Report. [Online] November 13, 2020. [Cited: November 16, 2020.] https://www.ecdc.europa.eu/sites/default/files/documents/influenza-characterisation-report-october-2020.pdf.
- Public Health Agency of Canada. November 8 to 14, 2020 (week 46). FluWatch. [Online] November 20, 2020. [Cited: November 21, 2020.] https://www.canada.ca/content/dam/phac-aspc/documents/services/publications/diseases-conditions/fluwatch/2020-2021/fw-week46-2020-en.pdf.
- CDC. Key Updates for Week 46, ending November 14, 2020. COVIDView: A Weekly Surveillance Summary of U.S. COVID-19 Activity. [Online] November 20, 2020. [Cited: November 21, 2020.] https://www.cdc.gov/coronavirus/2019-ncov/covid-data/pdf/covidview-11-20-2020.pdf.
- Olsen, Sonja et al. Decreased Influenza Activity During the COVID-19 Pandemic. Morbidity and Mortality Weekly Report. [Online] September 18, 2020. https://www.cdc.gov/mmwr/volumes/69/wr/mm6937a6.htm.
- Schultz-Cherry, Stacey. Viral Interference: The Case of Influenza Viruses. The Journal of Infectious Diseases. [Online] December 1, 2015. [Cited: November 11, 2020.] https://academic.oup.com/jid/article/212/11/1690/2911897.
- Huihui Yue MD, et al. The epidemiology and clinical characteristics of co-infection of SARS-CoV-2 and influenza viruses in patients during COVID-19 outbreak. Journal of Medical Virology. [Online] June 12, 2020. [Cited: November 11, 2020.] https://onlinelibrary.wiley.com/doi/10.1002/jmv.26163.
- Julia Stowe PhD, et al. Interactions between SARS-CoV-2 and Influenza and the impact of coinfection on disease severity: A test negative design. medRxiv: The Preprint Server for Health Sciences. [Online] September 22, 2020. This article is a preprint and has not been peer-reviewed. https://www.medrxiv.org/content/10.1101/2020.09.18.20189647v1.full.pdf.
- Daniela Marín‐Hernández, Robert E. Schwartz, Douglas F. Nixon. Epidemiological evidence for association between higher influenza vaccine uptake in the elderly and lower COVID‐19 deaths in Italy. Journal of Medical Virology. [Online] June 4, 2020. [Cited: November 11, 2020.] https://onlinelibrary.wiley.com/doi/full/10.1002/jmv.26120. Early View: Online version of record before inclusion in an issue.
- Günther Fink, et al. Inactivated trivalent influenza vaccine is associated with lower mortality among Covid-19 patients in Brazil. medRxiv: The Preprint Server for Health Sciences. [Online] July 1, 2020. [Cited: November 11, 2020.] https://www.medrxiv.org/content/10.1101/2020.06.29.20142505v1.full.pdf+html. Preprint.
- Priya A. Beisarun, et al. The effect of influenza vaccination on trained immunity: impact on COVID-19. medRxiv: The Preprint Server for Health Sciences. [Online] October 16, 2020. [Cited: November 11, 2020.] This article is a preprint and has not been peer-reviewed.. https://www.medrxiv.org/content/10.1101/2020.10.14.20212498v1.full-text.
- Claudio Zanettini, et al. Influenza Vaccination and COVID19 Mortality in the USA. PubMed.gov. [Online] June 16, 2020. [Cited: November 11, 2020.] Preprint: preprints have not been peer reviewed.. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7325191/.
- Wolff, Gregg G. Influenza vaccination and respiratory virus interference among Department of Defense personnel during the 2017–2018 influenza season. Vaccine. [Online] January 10, 2020. [Cited: November 11, 2020.] Epub 2019 Oct 10. PMID: 31607599; PMCID. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7126676/.