As the new Omicron variant spreads, experts say the best defense against all viral variants that cause COVID-19 is a full dose of a vaccine followed by a booster shot several months later.
Since the first mRNA vaccine against COVID-19 was authorized on December 11, 2020, studies have shown that unvaccinated people are at five times greater risk of getting infected and 10 times greater risk of hospitalization or death from COVID-19 than those who are fully vaccinated.
But research from Israel and the U.S. also revealed that vaccine-induced immunity against the SARS-CoV-2 coronavirus wanes in six to eight months. That’s especially worrying as more populations are exposed to the more contagious Omicron variant, which was first detected in November in South Africa.
To date Omicron has been reported in 77 countries. In the United States, the variant is in more than 37 states and makes up 3 percent of total cases; the rest are still Delta. But in London it now makes up the majority of positively diagnosed SARS-CoV-2 infections, and in South Africa it is the dominant strain.
Boosters, however, can restore antibody levels to their peak values—providing more robust protection against Omicron.
“Vaccines help to protect you, or at least prevent you from dying from the disease,” says Leo Poon, a virologist at the University of Hong Kong who detected some of the first cases of Omicron outside of South Africa. “And no matter what it is, Omicron or Delta, having a booster will be beneficial.”
What is a booster?
COVID-19 vaccines train our immune systems to make antibodies using synthetic versions of the virus’s spike protein—the part of the virus that helps it bind to human cells. If a vaccinated person later encounters the virus, the antibodies recognize it and bind to the spike protein to prevent infection.
The first dose of an mRNA vaccine prepares the cells to make antibodies, and the second dose matures and enhances those antibodies to bind even more strongly to the spike protein, so that it can’t anchor to receptors on human cells. In the case of the Johnson & Johnson vaccine, a single dose was enough to make sufficient antibodies against the original coronavirus.
But for all COVID-19 vaccines authorized to date, antibody levels do gradually decline, says Maria Elena Bottazzi, a vaccinologist at Texas Children’s Center for Vaccine Development at Baylor College of Medicine. That’s where boosters come in.
The booster dose is currently recommended by the U.S. Centers for Disease Control and Prevention for everyone ages 18 and older. The CDC says people should get boosted six months after their second dose of the Pfizer or Moderna mRNA vaccines, or two months after the J&J vaccine. The CDC has also recommended boosters for 16- and 17-year-olds who were fully vaccinated with two jabs of the Pfizer vaccine.
Scientists are still gathering evidence for how long immunity from the booster lasts and whether more will be needed down the line.
How does Omicron affect vaccines?
Since Omicron has accumulated over 30 mutations in the spike protein alone compared to the original virus, it seems to evade antibodies generated by two doses of the Pfizer or Moderna vaccines, or the single dose of the J&J vaccine, especially as antibody levels drop in the blood.
In a U.K. study that’s not yet been peer reviewed, the effectiveness of two shots of the Pfizer or AstraZeneca vaccines in preventing COVID-19 symptoms from Omicron fell to less than 40 percent within 15 weeks after the second dose. There was a lesser decline in the effectiveness of the Pfizer vaccine against Delta, but vaccine effectiveness still slipped to just 60 percent after 25 weeks.
Other preliminary studies from South Africa, Israel, and France also show steep declines in the ability of antibodies to neutralize Omicron in people vaccinated with the two-dose Pfizer vaccine or two doses of Moderna’s vaccine.
Boosters still work against Omicron
The good news is that a booster dose of the Pfizer vaccine increases antibody levels by 25-fold, which should be sufficient to neutralize Omicron. A booster dose of Moderna’s vaccine also improved the neutralization of Omicron compared to the previous two shots alone.
“The two doses with waning immunity mean there's no protection within a few months after the two doses,” says Peter Hotez, a pediatrician and vaccine scientist at Baylor College of Medicine. “The booster at least gives you something in the 70 percent range.”
Other studies show that when people received any mRNA booster dose, their antibody levels against Omicron rose to the protective level considered sufficient to prevent a COVID-19 infection.
“Our booster vaccine regimens work against Omicron,” Anthony Fauci said during a White House COVID-19 update on 15 December. “At this point, there is no need for a variant-specific booster,” Fauci added.
However, the data so far is largely from laboratory studies, and immunity involves more than just antibodies. More real-world data will be important to assess how effective current vaccines will be against Omicron in the long run.