The period between getting the Moderna COVID-19 vaccine and the first booster shot has been shortened to five months from six for people ages 18 and up, the Food and Drug Administration says.

The FDA's announcement Friday comes as the highly contagious omicron variant is spreading rapidly around the country and immunity from the first round of vaccines is fading. Over the weekend, more than 1 million people in the U.S. were diagnosed with COVID-19.

"Vaccination is our best defense against COVID-19, including the circulating variants, and shortening the length of time between completion of a primary series and a booster dose may help reduce waning immunity," said Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, in a statement.

Marks also said the change in wait time provides some consistency among some of the vaccines. On Monday, the FDA shortened the interval between the second Pfizer dose and booster to five months as well. For those who got the single-dose Johnson & Johnson vaccine, the time between getting the vaccine and booster remains at two months.

Moderna's first booster shot's efficacy is expected to last through the winter, the company's CEO said at a health care conference hosted by Goldman Sachs on Thursday, but by next fall the protection may start to dip again and a fourth shot of the vaccine would be necessary.

"I would expect that it's not going to hold great. ... I worry about next fall," Moderna CEO Stéphane Bancel said.

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