Rachael Rettner

COVID-19 may eventually become a seasonal illness like the flu, but only when the population achieves herd immunity, meaning a sufficient number of people are immune to prevent constant spread, a new study suggests.

But until then, COVID-19 will likely spread year-round, a finding that highlights the importance of following public health measures to control the virus, according to the review, published Tuesday (Sept. 15) in the journal Frontiers in Public Health.

“COVID-19 is here to stay and it will continue to cause outbreaks year-round until herd immunity is achieved,” study senior author Hassan Zaraket, of the American University of Beirut in Lebanon, said in a statement. “Therefore, the public will need to learn to live with it and continue practicing the best prevention measures, including wearing of masks, physical distancing, hand hygiene and avoidance of gatherings.”

What makes a virus seasonal?

Many viruses seem to follow seasonal patterns — for example, in temperate regions, cases of the flu regularly peak in winter and dwindle during the summer months. The same is true for certain types of coronaviruses that cause the common cold.

Scientists don’t know for sure why these viruses follow a seasonal pattern, but a number of factors are thought to play a role. For example, studies suggest that many respiratory viruses are more stable and linger in the air longer in environments with cold temperatures and low humidity, the authors said. Human behaviors, such as gathering indoors in wintertime, could also boost transmission.

Early studies on SARS-CoV-2, the virus that causes COVID-19, also suggested that the virus’s transmission may increase in colder temperatures and decrease in warmer temperatures.

But with any infectious disease, in order for cases to decline, a factor known as the “basic reproduction number” (R0, pronounced R-nought), or the average number of people who catch the virus from a single infected person, needs to drop below 1.

The R0 for COVID-19 appears to be relatively high, with many scientists estimating a value between 2 and 3, compared with about 1.3 for the flu, the authors said.

COVID-19’s high R0 may be due, in part, to the absence of pre-existing immunity to the disease in most of the population. Thus, with a higher R0, the authors predict it will be harder for seasonal factors to push R0 below 1.

“Therefore, without public health interventions, SARS-CoV-2 will continue to spread in summer as witnessed in many countries around the world,” the authors wrote.

In contrast, as more people gain immunity, either through natural infection or vaccine, the R0 “is expected to drop substantially, making the virus more prone to seasonal fluctuations,” such as spikes in wintertime and dips in summertime, the authors concluded.

Originally published on Live Science.