MARSHFIELD – Marshfield Clinic Research Institute was a key part of a Centers for Disease Control and Prevention-led study that found household spread of flu during the second flu season of the COVID-19 pandemic was more than twice as high as it was prior to the pandemic.

The CDC study published Thursday in the Journal of the American Medical Association looked at household flu transmission in the two flu seasons prior to the COVID-19 pandemic and the 2021-22 flu season among cohorts in four states, including Wisconsin.

During the 2021-22 season, 50% of household contacts of a patient with flu were themselves infected within seven days after the first person became sick, compared with 20% of household contacts during two flu seasons before the COVID-19 pandemic. The secondary spread of flu in the household setting estimated during the 2021-22 season was also higher than estimates reported from previous flu household transmission studies.

“This study highlights the need for household measures to prevent flu transmission, and the importance of annual flu vaccination to reduce the risk of flu illness,” said Research Scientist Huong McLean, Ph.D., one of the study’s authors and the local principal investigator.

Potential reasons for the higher levels of household spread of flu during the 2021-22 season compared to previous seasons include:

  • Antibodies against flu may have declined or waned as a result of lower levels of flu circulation in the two years after the start of the COVID-19 pandemic.
  • Reductions in seasonal flu vaccination among children during the COVID-19 pandemic may have contributed to increased population susceptibility.
  • Social interactions in the United States changed during the COVID-19 pandemic, with people generally spending more time at home, potentially raising the risk of household transmission.

All pre-pandemic study participant enrollments were done through Marshfield Clinic Research institute and Vanderbilt University. MCRI’s Integrated Research & Development Laboratory performed laboratory testing to identify influenza cases in the pre-pandemic seasons.

Joining McLean as authors of the study were Epidemiologist Dr. Ed Belongia, M.D.; and Jennifer Meece, Ph.D., Director of Integrated Research & Development Laboratory and MCRI interim executive director.