The U.S. Centers for Disease Control and Prevention recommends a second COVID-19 vaccine, six months apart, for the elderly and the immunocompromised.

The CDC recommendations suggest a second COVID-19 vaccine after six months for adults 65 and older and those 6 months to 64 years old who are “moderately or severely immunocompromised.”

“Data shows that vaccination is our best defense against severe cases of COVID-19 and is especially important in protecting the most vulnerable in our community,” Courtney Ronner, a Denver Department of Public Health and Environment spokesperson, said in an email to The Denver Gazette.

Ronner added: “As we know, you can contract COVID-19 year-round, so it makes sense to have continued coverage for high-risk groups.”

Since scientists first identified COVID-19 in December 2019. The novel virus has killed 7 million people worldwide, including 1.2 million Americans and 15,790 Coloradans.

While epidemiologists initially assumed COVID-19 would settle into becoming a seasonal disease much like the flu or RSV, the rising number of infections over the summer months suggest otherwise.

Typically, the flu circulates in the U.S. from October to May.

While the flu can be detected year-round, it usually peaks between December and February.

With the introduction of novel coronavirus in 2020, health officials now track COVID-19 in addition to flu and RSV, a common respiratory virus that triggers mild cold-like symptoms but can cause serious illness in infants and older adults.

Cold, dry winter months — when people tend to spend more time indoors —create more favorable conditions for respiratory viruses to remain stable, increasing the chances of transmission.

These, and other factors, contribute to the increased viral circulation during the winter months.

Flu cases plummeted across Colorado and the nation, denied transmission routes in 2020 after officials closed or restricted public schools and businesses during the pandemic. In the 2020-2021 respiratory season, Colorado had just 34 flu-related hospitalizations compared to more than 3,500 the year before.

Flu hospitalizations have been relatively flat since June, state data shows.

That’s not been the case for COVID-19.

Since June, Colorado has seen hospital admission rates — which typically lags about two weeks behind infections — rise from 1.5 per 100,000 hospitalizations to a peak of 4.8 the week of Sept. 7.

Hospital rates for COVID-19 — public health data shows — have been falling ever since.

Immunity can be achieved either through vaccination or prior infection.

“Because there was so much (Covid) circulating consistently for four years, I think what happened is you will naturally see the bumps and spikes,” Leslee Warren, an epidemiology unit supervisor for Denver Department of Public Health and Environment, has said.

“We haven’t had a season where it has completely fizzled out.”