Levels of influenza and other non-COVID-19 respiratory viruses were at historic lows during most of 2020, according to a new report from the Centers for Disease Control and Prevention. There’s still very little flu circulating, but other viruses — including parainfluenza viruses and common human coronaviruses, which cause colds — are having an out-of-season resurgence in 2021.

Between October 2020 and May 2021, levels of the flu in the United States were at their lowest since 1997, the first year there’s flu season data available, the analysis found. There was very little flu reported all over the world, and experts think the protective measures people took against COVID-19 — masking, distancing — suppressed the virus.

The nearly nonexistent flu season this year might mean that this fall and winter’s flu season could be more severe, the CDC report warned. Because there wasn’t much influenza around, people may not have been exposed to the virus at the same rates they usually are. That could blunt the normal levels of immunity to the virus. “Lower levels of population immunity, especially among younger children, could portend more widespread disease and a potentially more severe epidemic when influenza virus circulation resumes,” the authors of the report wrote. That means it’ll be especially important for doctors and nurses to encourage anyone older than six months to get their flu shots this fall, they said.

The circulation of respiratory syncytial virus (RSV), a virus that causes colds in adults but can be dangerous for infants, was also muted during 2020 and early 2021. Rates started to tick back up in April 2021, which is unusual — normally, levels of that virus peak in January. Parainfluenza viruses and common human coronaviruses followed a similar trend: levels were low through 2020 and then started to climb in February 2021.

The public health measures used to slow the spread of COVID-19 likely helped suppress these viruses over 2020, and they bounced back as communities in the US started to lift some of those restrictions. It’s still not clear exactly how the flu and various cold-causing viruses respond to different strategies used against COVID-19, so the trends in these viruses might be unpredictable over the next year as efforts to fight the pandemic continue.

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

The uptick in viruses that cause the common cold might also make it harder for people and their doctors to differentiate between COVID-19 symptoms and symptoms of other illnesses. During 2020, any cold- or flu-like symptoms were likely to be COVID-19 — it was one of the only viruses around. Now that other viruses are on their way back, the picture could be murkier.