Influenza A Outbreak Highlights Need for Strategies to Reduce Healthcare System Burden

An outbreak of influenza in Michigan represented part of the first substantial influenza activity during the COVID-19 pandemic, leading to an investigation in order to identify and control the sudden upsurge.

In November 2021, a rapid increase in influenza A(H3N2) cases detected by the University Health Service (UHS) at the University of Michigan in Ann Arbor was reported to the Michigan Department of Health and Human Services (MDHHS).

Led by Miranda J. Delahoy, PhD, a team of investigators identified 754 laboratory-confirmed influenza A cases and noted the importance of increasing vigilance for influenza in the upcoming winter season, particularly due to the substantial impact of COVID-19 on healthcare systems.

The Study

As of August 2021, those with COVID-19-like or influenza-like illness evaluated at UHS received testing for SARS-CoV-2, influenza, and respiratory syncytial viruses by rapid multiplex molecular assay. Data on demographics, genetic characterization of viruses, and influenza vaccination history data were reviewed.

Between October - November 2021, out of 3,121 persons tested, 745 (23.9%) received a virus test result positive for influenza A, 137 (4.4%) for SARS-CoV-2, and 84 (2.7%) for respiratory syncytial virus.

Data show >95% of influenza cases were detected between November 1 - 19, which suggested rapid spread with a single patient hospitalized. The median age was 19 years and consisted of 54.1% female, 60.0% off-campus students, and 34.6% living in on-campus residence halls.

From 380 specimens sequenced for influenza, all viruses consisted of the A(H3N2) 2a.2 subgroup.

Delahoy and colleagues noted that among 2,405 patients who received testing for influenza A during October - November, 128 of 481 (26.6%) with positive influenza test results and 512 of 1,924 persons (26.6%) with negative test results had receipt of 2021 - 2022 influenza vaccine ≥14 days before the test.

The Vaccine

The influenza vaccine provided protection against 4 different influenza viruses, including A(H1N1) pdm09, A(H3N2), B/Victoria lineage, and B/Yamagata lineage.

The study noted that historically, vaccine effectiveness has been lower against influenza A (H3N2) virus compared to others, potentially due to the rapid evolution of the virus allowing it to escape immunity.

Due to similar vaccination rates among those with positive and negative test results, investigators suggested that protection against mild infection with the 2a.2 subgroup of H3N2 viruses was low in this subgroup.

However, they also urged cautious interpretation of this finding due to the potential of incomplete vaccination history and changing coverage with current vaccination campaigns.

Patients also had mild influenza illness with vaccinations providing protection against outcomes including hospitalization and death, which are rare in the age group measured. Additionally, they noted the subgroup of H3N2 cannot be generalized to other age groups or high-risk populations and further investigation is needed to determine vaccine effectiveness.

The Takeaways

Due to the findings, the team highlighted the importance of increasing vigilance for mitigating severe influenza and the strain it puts on health care services.

They offered up several measures to do so, including improving influenza vaccination coverage in patients aged ≥6 months and diagnostic testing for influenza and SARS-CoV-2 infection for patients with acute respiratory illness.

“To help mitigate the potential severity of the influenza season, public health practitioners and clinicians should recommend and offer the current seasonal influenza vaccine to all eligible persons aged ≥6 months,” investigators wrote.

The Morbidity and Mortality Weekly Report (MMWR), “Influenza A(H3N2) Outbreak on a University Campus — Michigan, October–November 2021,” was published online by the CDC.