Increasing Flu Activity in Some States, Especially Among Young Adults

November 15, 2021—CDC’s most recent FluView reports increases in flu activity that could mark the beginning of the 2021-2022 flu season. While flu activity is still low overall nationally, there has been an increase of influenza A(H3N2) viruses detected in recent weeks. The majority of these have been found in young adults and children, who are commonly the drivers of community spread of flu. CDC also has received anecdotal reports of flu outbreaks among young adults, possibly attending colleges and universities in several states. In addition, the University of Michigan has reported a large and sudden increase in cases of flu among students on the Ann Arbor campus. CDC is working with the Michigan Department of Health and Human Services (MDHHS), Washtenaw County Health Department, and University of Michigan (U-M) to investigate the outbreak.

The occurrence or timing of these recent flu outbreaks is not particularly unusual. Flu activity often starts to increase in October, most commonly peaks in February and can last into May. The recent outbreaks and associated increasing detection of H3N2 viruses are notable because they represent the first significant flu activity of the 2021-2022 season (and the first since March 2020). This increase in flu activity as the holiday season is about to begin underscores the importance of flu vaccination. On college campuses, flu viruses are known to spread rapidly in close quarters like common living spaces, classrooms, shared restrooms, and through social activities, and yet young adults consistently have the lowest flu vaccination coverage every flu season in the United States. Also, early in-season CDC estimates suggest that flu vaccine uptake among other groups so far is lower this season than last, including among children and pregnant people.

U-M reports that its University Health Service has diagnosed more than 500 cases of flu since October 6, 2021, prompting MDHHS to request epidemiologic assistance (an Epi-Aid) from CDC. An Epi-Aid allows CDC to provide rapid, short-term, onsite technical assistance from Epidemic Intelligence Service (EIS) officers and other experts to help local authorities respond to an urgent public health problem. The goals of this Epi-Aid will be to evaluate the scope of the outbreak, assess risk factors for infection, support infection control measures, and quantify vaccine uptake among cases, which may help give an early sense of flu vaccine effectiveness this season. These findings could help inform guidance for influenza responses on university campuses and in the community at large during the COVID-19 pandemic.

There was historically low flu activity during 2020-2021, likely due to strict COVID-19 prevention measures. Other respiratory viruses were similarly impacted by the COVID-19 pandemic but have since resumed normal circulation patterns. Experts have been watching closely to see whether flu virus circulation would resume. During the most recent three weeks (week ending October 23, 2021 through the week ending November 6, 2021), influenza A(H3N2) viruses have been reported by public health laboratories in seven of the 10 HHS regions (Regions 1, 3, 4, 5, 7, 8, and 9), which covers much of the country geographically (with the exception of Alaska, Arkansas, Idaho, Louisiana, New Jersey, New Mexico, New York, Oklahoma, Oregon, Texas, Washington, Puerto Rico, and the U.S. Virgin Islands), indicating flu circulation is beginning to resume. Influenza A (H3N2) predominant seasons often are also associated with more severe flu seasons, especially for older adults and young children.

Flu vaccination is the best way to prevent the spread of flu and widespread flu illnesses. CDC recommends everyone 6 months and older get an annual flu vaccine. Anyone who has not received a flu vaccine this season should get vaccinated now. A flu vaccine protects against four flu viruses, including an H3N2 virus that was updated for this season’s flu vaccines. COVID-19 vaccination will not protect against flu. Both COVID-19 and flu vaccines are needed this season as recommended.

It’s important to note that flu antiviral drugs can be used to treat flu illness. CDC recommends prompt treatment for people who have flu or suspected flu and especially for those who are at higher risk of serious flu complications. Antiviral drugs are not a substitute for getting a flu vaccine. While flu vaccine can vary in how well it works, a flu vaccine is the best way to help prevent seasonal flu and its potentially serious complications. Antiviral drugs are a second line of defense that can be used to treat flu if you get sick.

CDC is continuing to monitor the situation and will provide updates as needed.