As a summer wave of Covid-19 continues to hit the United States, the US Food and Drug Administration on Thursday signed off on updated Covid-19 vaccines from Moderna and Pfizer/BioNTech.

Moderna and Pfizer said updated mRNA shots will be available in pharmacies and clinics in the days ahead.

“Vaccination continues to be the cornerstone of COVID-19 prevention,” Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said in a statement. “These updated vaccines meet the agency’s rigorous, scientific standards for safety, effectiveness, and manufacturing quality. Given waning immunity of the population from previous exposure to the virus and from prior vaccination, we strongly encourage those who are eligible to consider receiving an updated COVID-19 vaccine to provide better protection against currently circulating variants.”

A CVS representative said doses are expected to start arriving at its locations within a few days. “Once vaccine supply arrives at one of our pharmacies, walk-ins will be accepted,” CVS’s Amy Thibault said. “Our digital scheduler is being updated with available vaccination appointments as well,” and customers can check for local availability online.

A spokesperson for Walgreens said its customers will be able to schedule appointments to get the updated vaccine within 24 hours, with availability starting September 6. However, as vaccine supply arrives in the coming days, walk-ins will also be available.

Levels of the SARS-CoV-2 virus, which causes Covid-19, measured in wastewater are at “very high” levels nationally, according to CDC data, sparking the highest summer peak in the US since July 2022. Monitoring of viral levels in wastewater can give a picture of how widespread the virus is as testing and other forms of monitoring have fallen off.

Measures of severe disease, including rates of hospitalization and death, have been rising, according to the CDC, but they’re nowhere near levels seen in previous years.

The prevalent strain in the US now is KP.3.1.1, according to CDC data, estimated to account for 37% of cases over the past two weeks. That’s triple its level a month ago.

KP.3.1.1 and KP.2 – the strain included in the updated mRNA vaccines – are both offshoots of JN.1, which will be the target of an updated protein-based vaccine from Novavax. All are versions of the Omicron variant.

The US Centers for Disease Control and Prevention recommended in June that everyone over 6 months old receive both an updated Covid-19 vaccine and a flu shot this year.

The FDA said Thursday that people 5 and older are eligible to receive an updated Pfizer or Moderna Covid-19 vaccine as long as it has been at least two months since their last dose.

Unvaccinated people age 6 months through 4 years can receive three doses of the updated Pfizer vaccine or two doses of the updated Moderna vaccine. People in this age group who have been previously vaccinated can receive one to two doses of the updated vaccines, depending on the timing and the number of doses they’ve previously received. Certain people ages 6 months through 11 years who have weakened immune systems may be eligible for additional doses.

CNN’s Katherine Dillinger contributed to this report.