The common cold was rare in 2020 – but it’s experiencing a resurgence

Levels of influenza and other non-COVID-19 respiratory viruses were at historically low levels for most of 2020, according to a new report from the Centers for Disease Control and Prevention. There is still very little flu circulating, but other viruses – including parainfluenza viruses and common human coronaviruses, which cause colds – are experiencing an off-season resurgence in 2021.

Between October 2020 and May 2021, flu levels in the United States were at their lowest since 1997, the first year that flu season data is available, according to the analysis. There has been very little flu reported worldwide, and experts believe the protective measures people have taken against COVID-19 – masking, distancing – have suppressed the virus.

The almost non-existent flu season this year could mean the flu season this fall and winter could be more severe, the CDC report warned. Because there wasn’t a lot of flu around, people may not have been exposed to the virus at the same rates they usually are. This could blunt normal levels of immunity to the virus. “Lower levels of population immunity, especially in young children, could portend more widespread disease and a potentially more severe epidemic when influenza virus circulation resumes,” wrote the authors of the report. This means that it will be especially important for doctors and nurses to encourage anyone over the age of six months to get a flu shot this fall, they said.

Image: Morbidity and Mortality Weekly Report

The circulation of respiratory syncytial virus (RSV), a virus that causes colds in adults but can be dangerous for infants, was also reduced in 2020 and early 2021. Rates began to rebound in April 2021, this which is unusual – normally, the levels of this virus peak in January. Common human parainfluenza viruses and coronaviruses followed a similar trend: levels were low until 2020, then started to climb in February 2021.

Public health measures used to slow the spread of COVID-19 likely helped suppress these viruses in 2020, and they have rebounded as communities in the United States have started to lift some of those restrictions. It is still unclear exactly how the flu and various cold-causing viruses respond to the different strategies being used against COVID-19, so the trends for these viruses could be unpredictable over the next year as efforts to combat the pandemic continues.

“Clinicians should be aware that respiratory viruses may not exhibit typical seasonal circulation patterns and that a resumption of circulation of some respiratory viruses is underway,” the CDC report said.

The rise in viruses that cause colds could also make it harder for people and their doctors to differentiate between symptoms of COVID-19 and symptoms of other illnesses. In 2020, all cold or flu symptoms were likely to be COVID-19 – it was one of the only viruses around. Now that more viruses are on their way back, the picture could be darker.