If you’re confused about whether you need a second COVID-19 vaccine booster or even which one to get, you seem to be in very good company. Even more confusing is when to roll up your sleeve.

The Centers for Disease Control and Prevention has recommended one booster for everyone 18 and older. A second booster has only been recommended for those 50 and older or for those who are at least 12 and immunocompromised.

Those who are eligible for a second booster are told to wait at least four months after their first booster before they can get a Pfizer-BioNTech shot or a Moderna shot. Teens who qualify are only eligible for the Pfizer-BioNTech version, according to the CDC guidance.

For people whose first vaccine was Johnson & Johnson’s, in most cases either Moderna or Pfizer are recommended for a first booster in adults, while those are the only brands acceptable for the second booster.

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With the emergence of new variants, Time’s Alice Park reported that “experts, however, aren’t comfortable with a strategy of simply adding booster after booster of the same vaccine. So they have launched studies to see if there is a better way to optimize the vaccines, and whether the current versions of the shots are the best ones to rely on in the face of a still-mutating virus.”

The questions are still being answered, but a group of advisors for the Food and Drug Administration looked at recent studies to try to answer the question. Research from Israel found that due to the emergence of variants, there’s some increase in illness among people who got a booster, but they’re not getting as sick as those who didn’t get the first booster.

The panel noted that immunity from the vaccines wanes, as much as a 25-fold drop against Omicron for those who were fully vaccinated but not boosted, and about 6-fold for those who were boosted.

Because of the potential for side effects and because a lot still isn’t known, the group said they don’t have enough evidence to recommend everyone get a second booster.

After that meeting, Time magazine reported, “For now, the vaccination schedule is a complicated algorithm depending on which vaccine people get, as well as their age and health status.”

Officials are considering a flu-shot model, where an annual shot is determined based on a best guess of the strain that’s likely to circulate. Time noted that’s not quite as clear with COVID-19, since the variants don’t necessarily follow the pattern of change seen with influenza.

“The issue of how we decide when the vaccine needs to be modified, and what is going to be the threshold where we say so much escape from vaccine immunity requires a change — that’s such a difficult question to answer,” said Dr. Cody Meissner, director of pediatric infectious disease at Tufts Medical Center, who is on the FDA committee, per Time.

Yale Medicine points out that booster eligibility continues to expand. Dr. Albert Shaw, one of Yale Medicine’s infectious disease specialists, said, “The main question is how long the immunologic protection against SARS-CoV-2, which causes COVID-19, lasts. And since we are learning about COVID-19 in real time, this is hard to know definitively.” 

The recommendation to get a booster doesn’t mean the existing vaccines are a failure, he said. Rather, it’s a result of what we continue to learn about how the virus works and what is an effective tool to curb the serious illness it can cause.

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The Los Angeles Times reported that the public health officials there definitely believe people who are immune-compromised or over 50 should get that second booster shot to reduce the possibility of having serious illness if one gets COVID-19.

“Evidence all over the world shows waning protection from the vaccines over time,” L.A. County Public Health Director Barbara Ferrer told the newspaper. “People who are at higher risk, people who are older, people who have underlying health conditions: Don’t delay.”

Last September, the World Health Organization’s science podcast explored the value of boosters. At the time, Dr. Katherine O’Brien, who is the director of the group's Department of Immunization, Vaccines and Biologicals, explained the circumstances that call for boosting the vaccine:

  • Some people who are immunocompromised don’t respond adequately to the original full vaccination doses.
  • Immunity might wane over time.
  • The original vaccine doesn’t perform as well against emerging variants.

All three of those factors appear to be in play at this stage of the pandemic. And the only thing certain is that the advice on boosters is evolving, experts say.

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