New research published by the Journal of the American Medical Association from Vanderbilt University Medical Center investigators found that the household spread of flu during the 2021-2022 season was more than twice as high as it was prior to the COVID-19 pandemic.
During that season, 50% of household contacts of someone with flu were themselves infected within 7 days after the first person became sick, compared with 20% of household contacts during two flu seasons before the COVID-19 pandemic, researchers found.
The increase in risk was highest among individuals age 12 years or older and those who did not report having a flu shot, the study found.
“Our VUMC team has worked closely with other academic institutions and with Centers for Disease Control and Prevention – CDC’s scientists evaluating the transmission of respiratory viral infections in households since 2017. This multicenter study is among the first to measure and compare influenza transmission in households during multiple seasons including the pandemic period” said Dr. Carlos G. Grijalva, MD MPH FIDSA, professor of Health Policy and Biomedical Informatics at VUMC, and senior author of the study.
Researchers noted there are a few possible reasons for the increased risk since the start of the pandemic, including changes in immunity against flu driven partially by the lower levels of flu circulation since 2020, changes in annual uptake of influenza vaccination in some groups and changes in how people congregated after the start of the pandemic.
The report also said that seasonal variation in risk of influenza infection has been reported prior to the COVID-19 pandemic, and that it is possible that the higher rates of infection observed during the 2021-2022 season may reflect these seasonal variations and would have occurred without the COVID-19 pandemic. The ongoing multicenter household transmission studies Co-led by VUMC investigators Dr. Grijalva and Dr. H. Keipp Talbot – associate professor of Medicine and Health Policy, may help clarify this possibility.
The investigators underscore that influenza vaccines remain the best available approach to protect individuals and their household members from the disease.
VUMC co-authors in the report include H. Keipp Talbot, MD, MPH, FIDSA; James D. Chappell, MD, PhD; Natasha B. Halasa, MD, MPH; Kimberly E. Hart, MA; Ahra Kim, MPH; Christopher J. Lindsell, PhD; Jonathan E. Schmitz, MD, PhD and Yuwei Zhu, MD, MS.
The research was funded by the CDC and the Food and Drug Administration.