December 16, 2021

First U.S. Flu Outbreak Since Pandemic Hits University

The University of Michigan in Ann Arbor was the site of the first substantial flu activity in the United States since the start of the COVID-19 pandemic, according to a recent CDC report. From Oct. 6 through Nov. 15, 745 (24 percent) of 3,121 people who were tested for respiratory illnesses were positive for influenza A, compared to 137 for SARS-CoV-2 and 84 for respiratory syncytial virus. Among those infected with flu, one patient was hospitalized.

The predominant strain of influenza in these individuals was a subgroup of A(H3N2). The A(H3N2) viruses are thought to evolve more rapidly than other strains and are therefore potentially better able to evade immunity. Updates to the flu vaccine in February 2021 were intended to protect against A(H3N2), including the subgroup driving the Michigan outbreak. But vaccination rates among those in Michigan who tested negative for flu were similar to those who tested positive, a finding that suggests the vaccine provides “low” protection against the A(H3N2) subgroup of concern, according to the report. Still, the vast majority of flu cases in Michigan were mild, and the vaccine is intended to protect against other outcomes in addition to mild illness, including hospitalization and death. These severe outcomes are rare in the demographic studied—the median age of those with positive flu tests was 19 years old—and the report’s authors caution that their findings are not generalizable to other age groups or high-risk populations. CDC continues to recommend that eligible people at least six months of age obtain the flu vaccine.

According to CDC, flu activity was “unusually low” in the U.S. during 2020–2021, a period that overlapped with a surge of SARS-CoV-2 infections. Between September 2020 and May 2021, only 0.2 percent of respiratory specimens tested for a flu virus were positive, compared to a range of 26 percent to 33 percent in the three previous flu seasons. Flu vaccination rates during the 2020–2021 season were similar to those of previous seasons. Possible explanations for the low incidence of flu in 2020–2021 include the wearing of face masks, increased hand washing, school closures, better ventilation of indoor spaces, physical distancing, and other measures adopted to mitigate the COVID-19 pandemic. The CDC report suggests that adoption of these measures, in addition to increased vaccination rates among eligible populations, could help control flu.