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Elon Musk casts doubt on whether a 2nd COVID booster helps or hurts. Here’s what studies show.

Elon Musk
Elon Musk. (Dimitrios Kambouris/Getty Images for the Met Museum/Vogue)

Twitter owner and CEO Elon Musk waded into a discussion about COVID-19 boosters in a tweet on Wednesday, with the billionaire claiming that it “isn’t clear whether, all things considered, a second booster helps or hurts.”

Despite a surge in COVID-19 cases nationwide and waning immunity against the disease, Musk’s remark comes at a time when less than 40% of people age 65 and older have received a bivalent booster, according to the CDC, with officials blaming misinformation for the low number of vaccinations.

Is a second COVID-19 booster effective?

Updated bivalent boosters designed to protect against both the original coronavirus strain and circulating Omicron variants became available in September for people 12 and older, in October for children 5-11 and in December for children 6 months to 4 years old who completed the Moderna vaccine primary series.

The Centers for Disease Control and Prevention’s recommendations for who should get a second booster — or fourth inoculation of an mRNA Pfizer or Moderna vaccine — vary depending on age and whether a person is moderately or severely immunocompromised. Available data suggests that while bivalent boosters may not offer strong protection against breakthrough infections, they do help protect against severe disease, hospitalization and death — especially in vulnerable populations.

Prior to the approval of the bivalent booster, World Health Organization experts said in a statement last August that “there is increasing evidence on the benefits of a second booster dose of vaccines in terms of restoring waning vaccine effectiveness.”

“Evolving evidence from studies suggests that additional protection of the most vulnerable populations, at least for several months, is likely to be achieved through administration of a second booster dose, although follow-up time for these studies is limited,” the statement continued.

With the new bivalent boosters targeting the COVID-19 strain responsible for a majority of cases in the U.S., experts had hoped they could better protect against infection. However, two studies, one by researchers at Columbia University and the University of Michigan and the other by researchers at Beth Israel Deaconess Medical Center in Boston, found that the bivalent boosters produced an antibody response comparable to but not better than the original monovalent vaccines — meaning they reduced incidence of death from COVID as well as the severity of illness, but did not reduce the likelihood of transmission.

But for older, more vulnerable populations, a bivalent booster may be effective at reducing severe outcomes from infection. An Israeli study on Pfizer bivalent boosters that was published last week but has not yet been peer-reviewed found that participants 65 and older who received the bivalent vaccine had significantly lower hospitalization and mortality rates from COVID-19 up to 70 days after vaccination than those who did not get boosted.

A study involving nursing home residents published in September found that “receipt of a second mRNA COVID-19 booster dose during circulation of SARS-CoV-2 Omicron subvariants was 74% effective at 60 days against severe COVID-19-related outcomes (including hospitalization or death) and 90% against death alone compared with receipt of a single booster dose.”

For adults 18 and older, a report by the CDC found that bivalent booster doses “provided additional protection against COVID-19-associated emergency department/urgent care encounters and hospitalizations in persons who previously received 2, 3, or 4 monovalent vaccine doses.”

Are the COVID boosters safe?

President Biden receiving his third COVID-19 booster
President Biden receiving his third COVID-19 booster, Oct. 25, 2022. (Tom Brenner for the Washington Post)

Common side effects reported after getting a booster are similar to those reported after a primary shot, and are usually mild and temporary, including fatigue or pain at the injection site. The CDC says serious side effects that could cause a long-term health problem are extremely rare following any vaccination, “including COVID-19 vaccination.”

When bivalent boosters were first given emergency-use authorization by the Food and Drug Administration, some were concerned about the lack of human clinical trial data, but updated information from clinical trials has since become available. In November, Pfizer released data showing that the bivalent booster’s safety in human adults was favorable and similar to its original monovalent COVID-19 vaccine. Moderna reported in November that it had no new safety concerns in its human clinical trials.

A recent study funded by the European Research Council also supported the safety of a second Pfizer booster, and found that “the second booster was not associated with any of the 25 adverse events investigated.”

Health officials have been trying to overcome vaccine hesitancy since the beginning of the pandemic, a problem that has been exacerbated by an uptick in misinformation on Twitter. Shortly after Musk acquired the social media platform, the company announced that as of Nov. 23 it would no longer enforce the company’s COVID-19 misleading-information policy, which involved labeling or removing content with misinformation about COVID vaccines. Though Musk opposes vaccine mandates, he told Time magazine in 2021 that he and his eligible children were vaccinated and that “the science is unequivocal.”