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Americans who received the Pfizer-BioNTech or Moderna coronavirus vaccines should get a booster shot eight months after their second vaccine dose, federal health officials said Wednesday.

The boosters will be available beginning Sept. 20, if the Food and Drug Administration agrees to the plan. They will go first to health care workers, nursing home residents and older adults, who were the first to receive the initial round of vaccinations after they were authorized in December.

But the recommendation does not apply to the nearly 14 million Americans who received the single-shot Johnson & Johnson vaccine.

“For people who received the Johnson & Johnson vaccine, we anticipate vaccine boosters will likely be needed,” Dr. Vivek Murthy, the surgeon general, said at a White House news briefing Wednesday.

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“We expect more data on J&J in the coming weeks," he added. "With those data in hand, we will keep the public informed of a timely plan for J&J booster shots.”

In the meantime, here are answers to some common questions.

Why didn’t officials recommend boosters for people who got the Johnson & Johnson vaccine?

All of the vaccines authorized in the United States provide strong protection against severe disease and death from COVID-19. But the booster recommendation was based on data suggesting that the protection provided by the mRNA vaccines against infection and mild disease has been waning over time, officials said Wednesday.

“Even though this new data affirms that vaccine protection remains high against the worst outcomes of COVID,” Murthy said at the briefing, “we are concerned that this pattern of decline we are seeing will continue in the months ahead, which could lead to reduced protection against severe disease, hospitalization and death.”

Less data is available on the Johnson & Johnson vaccine, which was not authorized until the end of February, two months after the mRNA vaccines. (The Johnson & Johnson vaccine uses a modified adenovirus to deliver its instructions to human cells.) In addition, Johnson & Johnson vaccinations were temporarily paused while health officials investigated reports that a very small number of people had developed a rare blood-clotting condition after receiving the vaccine.

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More than 150 million Americans have gotten mRNA vaccines, far exceeding the 14 million who have received the Johnson & Johnson shot, according to data from the Centers for Disease Control and Prevention.

Without robust data on the long-term effectiveness of the Johnson & Johnson vaccine, it is hard for health officials to recommend boosters, said John Moore, a virologist at Weill Cornell Medicine. “If you’re doing data-driven decisions and you don’t have the data, what can you do?” he said. “This is sort of the dilemma. Public confidence in vaccines generally depends on seeing how the sausage is made, seeing that it is a data-driven, transparent process.”

How effective is the Johnson & Johnson vaccine?

Clinical trials, conducted before the delta variant was widespread, found that the Johnson & Johnson vaccine had 72% efficacy overall in the United States, lower than the roughly 95% efficacy of the Pfizer and Moderna vaccines. However, it is hard to make direct comparisons between the vaccines, which were tested in different locations and at different times.

All of the available vaccines appear to lose some effectiveness against delta, which may be able to dodge some of the immune system’s antibodies. The Johnson & Johnson vaccine is no exception. “You would expect there to be a degree of resistance against delta, because there always is,” Moore said.

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Small laboratory studies have turned up contradictory results on how well the Johnson & Johnson vaccine protects against delta. Last month, Johnson & Johnson said that a single dose of its vaccine prompted a strong immune response against delta and that the response lasted at least eight months.

But data from another recent laboratory study suggested that a single dose of the vaccine elicited a relatively weak antibody response against delta, which could make boosters more essential.

The first real-world data on the vaccine’s efficacy against the variant was released this month. The data, which are preliminary results from clinical trial of nearly 500,000 health care workers in South Africa, suggested that a single dose of the vaccine had efficacy of up to 96% against death and 71% against hospitalization from infections caused by delta.

It was “a very large analysis and very clear results showing that the single-shot J&J vaccine provided substantial protection against the delta variant,” said Dr. Dan Barouch, a virus expert at Beth Israel Deaconess Medical Center in Boston who has conducted studies for Johnson & Johnson but was not involved in the South Africa trial.

When will we know more?

Soon, the company said. “We are engaging with the FDA, CDC and other health authorities and will share new data shortly regarding boosting with the Johnson & Johnson COVID-19 vaccine,” Johnson & Johnson said in an emailed statement.

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A large clinical trial of the safety and efficacy of two doses of the vaccine is underway, and data will be available “in the coming weeks,” Johnson & Johnson said.

Will I eventually need a booster?

Federal health officials and outside experts said that they expected that boosters would ultimately be recommended for people who had received the Johnson & Johnson vaccine.

“Given the hyperinfectious delta variant, it is likely that people who received the J&J vaccine will be recommended for a booster shot at some point in time,” Barouch said.

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Moore agreed, adding that he expected that recommendation to come sooner rather than later.

“I would be very, very surprised if that did not happen in the reasonably near future,” Moore said. He added, “I can’t pluck a date out of the air, but I know it’s on the radar screen.”

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When will I need a booster?

It’s not clear yet. “The timeline for boosters will be determined in part by data that I expect to come out over the next several weeks,” Barouch said.

The first Johnson & Johnson vaccines were not given until early March. If federal officials recommend the same eight-month timeline between vaccination and booster shots, the first Johnson & Johnson boosters would most likely start in November, Barouch said.

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Will the booster be another Johnson & Johnson shot?

Some scientists have suggested that mixing and matching vaccines may provide better protection than getting two doses of the same shot. And research suggests that people who follow a single dose of the AstraZeneca vaccine, which uses technology similar to the Johnson & Johnson shot, with the Pfizer vaccine have a more robust immune response than those who get a second dose of AstraZeneca.

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Some people who have received the Johnson & Johnson vaccine are seeking mRNA boosters on their own. San Francisco health officials have said that they will accommodate these requests as long as people consult with their doctors first.

Although Barouch said he could not predict what kind of booster federal officials might ultimately recommend for people who had gotten the Johnson & Johnson vaccine, the data he expects to be released soon is from studies of two doses of the Johnson & Johnson vaccine, rather than an mRNA booster.

Indeed, Moore anticipated that if the government recommended a booster in the near future for people who had received the Johnson & Johnson vaccine, it would be for a second dose of the same vaccine. “There won’t be any data on J&J first, mRNA second,” he said.

What should I do in the meantime?

Sit tight, experts said. “The federal government is well aware of the J&J situation,” Moore said. “It’s not being overlooked.”

Barouch agreed. “I do expect a substantial amount of data to come out in the next few weeks,” he said. “So, I understand that people who received the J&J vaccine want more information, and that information will be coming.”

This article originally appeared in The New York Times.