• The Centers for Disease Control and Prevention signed off on Novavax's COVID-19 vaccine on Tuesday.
• In a statement, the CDC said the shots will be available in the coming weeks. The U.S. has secured 3.2 million doses so far.
• Novavax uses more conventional technology than Pfizer and Moderna, and U.S. health officials hope Novavax's shots might convince skeptics to get vaccinated.
• 26 million to 37 million adults are still unvaccinated in the U.S., according to CDC estimates.

The Centers for Disease Control and Prevention on Tuesday signed off on Novavax's two-dose COVID-19 vaccine as a primary series for adults, offering people who are unvaccinated a choice to receive a shot based on conventional technology in use for more than 30 years.

The CDC's committee of independent advisors voted unanimously to recommend the vaccine for people ages 18 and older after reviewing the shots' safety and effectiveness during an hours-long public meeting Tuesday. CDC Director Dr. Rochelle Walensky backed the recommendation later in the evening, the final step in the U.S. authorization process.

The CDC, in a statement, said the vaccine will be available to the public in the coming weeks. The Biden administration has secured 3.2 million doses of Novavax's vaccine so far, according to the Health and Human Services Department.

The CDC's approval completes a two-year journey for Novavax, one of the early participants in the U.S. race to produce a vaccine to protect against COVID-19. The small Maryland biotech company received $1.8 billion of taxpayer money from Operation Warp Speed, but struggled to get its manufacturing base in place and ultimately fell behind Pfizer and Moderna.

Novavax's vaccine will enter the U.S. at a time when more than three-fourths of U.S. adults, 77%, are now fully vaccinated with Pfizer, Moderna and to a much lesser degree Johnson & Johnson's shots, according to CDC data.

Walensky, in a statement, said the Novavax vaccine will provide another option for people who do not want to take Pfizer's and Moderna's shots. Anywhere from 26 million to 37 million adults are still unvaccinated in the U.S., according to CDC data, but it's unclear how many of those people will opt to take Novavax's vaccine.

"The primary target population for Novavax will be the 10% to 13% of those that are unvaccinated," said Dr. Oliver Brooks, a CDC vaccine committee member and chief medical officer at Watts HealthCare Corporation in Los Angeles.

"I understand we're really focused on that population with the hope that perhaps this protein subunit vaccine will change them over from being unvaccinated to vaccinated," Brooks said.

Effectiveness and safety

Novavax's shots were 90% effective at preventing illness overall and 100% effective at preventing severe disease when the alpha variant of COVID was dominant, according to clinical trial data. The company has not published data on the real-world effectiveness of its shots against omicron and its subvariants.

But Novavax's vaccine appears to produce a strong immune response against omicron and its subvariants, which would suggest that it is effective against the variant. The company's immune response data against omicron impressed members of the Food and Drug Administration's vaccine committee last month.

The FDA in a fact sheet for health-care providers warned that Novavax's vaccine appears to carry a risk of heart inflammation called myocarditis. FDA officials flagged four cases of myocarditis and pericarditis — swelling in a membrane surrounding the heart — from Novavax's clinical trial in young men ages 16 to 28.

Pfizer and Moderna's shots also pose an elevated risk of myocarditis for adolescent boys and young men after the second dose. The CDC has found that COVID infection carries a greater risk of heart inflammation than vaccination with Pfizer or Moderna's shots. Myocarditis is typically caused by viral infections.

Different technology

Novavax's vaccine uses a different platform than Pfizer's and Moderna's shots. The Novavax vaccine is based on protein subunit technology in use for more than 30 years, starting with the first licensed hepatitis B vaccine, according to the CDC. The technology is also used in vaccines for the flu and whooping cough.

Novavax produces inactive copies of the virus spike protein by inserting genetic code into an insect virus that infects moth cells. The spike copies are harvested and purified from those cells for its vaccine.

The spike protein is the mechanism the virus uses to invade human cells. The spike copies in the vaccine prepare the body's immune system to produce antibodies that block the virus from invading human cells. Novavax's vaccine also has an additional ingredient called an adjuvant that is derived from the bark of a South American tree to produce a broader immune response to fight the virus.

Pfizer and Moderna's vaccine, by contrast, use messenger RNA technology. Their shots deliver messenger RNA to human cells, which then produce inactive copies of the spike protein to generate an immune response to fight the virus. Pfizer and Moderna's shots are the first mRNA vaccines to receive approval from the FDA.

Novavax's shots are also stored at normal refrigerator temperatures, while Pfizer's and Moderna's shots require subzero cold.

Though the FDA has repeatedly authorized Pfizer's and Moderna's shots as safe and effective vaccines for every age group in the U.S., health officials have struggled to convince millions of skeptics to take the shots.

Fall vaccine change

The U.S. is planning to tweak the vaccines to target the more transmissible BA.4 or BA.5 omicron subvariants, which are now dominant, ahead of an expected fall vaccination campaign. The COVID vaccines are less effective at preventing infection from omicron due to the variant's many mutations, but they are still generally protecting against severe disease.

U.S. health officials believe changing the formula of the vaccines to target omicron variants as well as the original strain that emerged in Wuhan, China, in 2019 will provide more durable protection against the virus this fall.

Novavax's vaccine, like every currently authorized COVID shot, is based on the original strain of the virus that first emerged in China. Like Pfizer and Moderna, Novavax is developing vaccines that target the omicron variant, but company executives have said it's not clear that they need to roll out an updated shot because the company's current vaccine triggers a broad immune response against the variant.

The CDC and the FDA have not approved mixing and matching Novavax's vaccine with Pfizer's and Moderna's shots as a booster. Novavax has said it plans to ask the FDA to authorize a booster shot of its vaccine.