Citing the rapid spread of the omicron variant and the need for protection against it, U.S. federal health officials are shortening the recommended time between the second dose of the Moderna vaccine and a booster shot from six months down to five.

"The country is in the middle of a wave of the highly contagious omicron variant, which spreads more rapidly than the original SARS-CoV-2 virus and other variants that have emerged," Peter Marks, M.D., director of the U.S. Food and Drug Administration Center for Biologics Evaluation and Research, said in a statement.

"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," he said. "Today's action also brings consistency in the timing for administration of a booster dose among the available mRNA vaccines. We encourage everyone to get vaccinated -- it's never too late to get your COVID-19 vaccine or booster."

The new recommendation applies to everyone aged 18 years and older. The FDA notes that side effects from the booster shot are typically mild -- some discomfort at the site of injection and transient fatigue, headache, or muscle aches.

A similar time window for boosters is already in place for people who received the Pfizer/BioNTech vaccine.