(CNN) — Novavax’s Covid-19 vaccine is now available in the United States for use as a first booster dose for people 18 and older.

The US Food and Drug Administration authorized the booster shot Wednesday for adults who are at least six months past a primary Covid-19 vaccine series, and the US Centers for Disease Control and Prevention recommended its use.

The company said its booster is for adults for whom a Pfizer/BioNTech or Moderna updated booster vaccine is not accessible or clinically appropriate, and for adults who elect to receive the Novavax vaccine because they would otherwise not get a booster dose of a Covid-19 vaccine.

Previously, the Novavax vaccine was used only as a two-dose primary series. It is the fourth coronavirus vaccine available in the United States, and unlike the others, it uses a protein-based technology.

“The U.S. now has access to the Novavax COVID-19 Vaccine, Adjuvanted, the first protein-based option, as a booster,” Novavax President and CEO Stanley Erck said in a news release Wednesday. “According to CDC data, almost 50 percent of adults who received their primary series have yet to receive their first booster dose. Offering another vaccine choice may help increase COVID-19 booster vaccination rates for these adults.”

In July, the FDA authorized Novavax’s Covid-19 vaccine for use as a two-dose primary series in adults, and the authorization was expanded in August to ages 12 to 17.

Novavax executives have long suggested that their Covid-19 vaccine could be used as a booster dose, even for people who got another vaccine type as their primary series.

Novavax has been a late comer on the vaccine scene in the US, though it has been widely used in many other countries, including as a booster dose in Europe, Japan, Australia, New Zealand, Switzerland and Israel.

“It’s important because globally, close to 70% of our doses are being used in boosting doses, and that’s because everyone’s already been vaccinated. So that’s really where the opportunity to help protect people lies. So that’s why we’re excited about this in the US,” Dr. Filip Dubovsky, chief medical officer of Novavax, told CNN.

The vaccine is based on more traditional technology than the mRNA vaccines from Pfizer and Moderna or the adenoviral vectors used in the Johnson & Johnson vaccine.

Protein-based vaccines like the Novavax shot teach the immune system to recognize little modified pieces of the virus that the vaccine is targeting. In this case, that means fragments of the coronavirus spike protein. The vaccine was created out of a genetic sequence of the original strain of the coronavirus.

The company presented data last week at the World Vaccine Congress showing that when its vaccine is used is a booster, it elicits a wide range of antibodies against many Omicron subvariants, including BA.1, BA.2 and BA.5.

Dubrovsky said the company believes that its vaccine generates a broad antibody response because of the type of technology it uses. He says the company has not seen any data that suggests that it needs to update the vaccine to include more current strains like BA.4 and BA.5.

“All the data right now points to us that our current vaccine, the one that’s been used, when it has a very extensive safety database, seems to work just fine against the variants that are circulating right now,” he said.

In the clinical trials, side effects were mild and lasted about two days. The most common side effects reported with the Novavax booster were pain or tenderness at the injection site, fatigue, muscle pain and headache.

As of Monday, about two-thirds of the US population (68%) is fully vaccinated with at least their initial series of Covid-19 vaccine. But only about a third (33.5%) of the population has received a booster dose of vaccine.

Health officials were hopeful that offering Novavax as an option might persuade those who had been hesitant to get vaccinated against Covid-19. But by the time of its July authorization, most Americans had received their primary Covid-19 vaccines, and use of Novavax in the US has been sluggish.

An update to the CDC’s vaccine advisers in September showed that only 14,449 Novavax doses had been administered by the end of August, and only about 2,600 Americans had completed a two-dose primary series of the vaccine.

FDA, CDC sign off on Novavax Covid-19 vaccine as a first booster shot