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As new strains complicate vaccine picture, Moderna plans studies of a COVID-19 booster shot

The Cambridge biotech says its existing vaccine should still protect against the emerging variants

Registered nurse Samantha Schuko drew up a dose of the Moderna vaccine during vaccinations for first responders at Gillette Stadium recently.Jessica Rinaldi/Globe Staff

Moderna said Monday that its COVID-19 vaccine generates a weaker immune response against a worrisome strain of the disease that first emerged in South Africa, prompting the company to plan a US clinical trial of a “booster” shot targeting the variant.

The announcement by the Cambridge biotech underscores that even as government officials strive to speed up the nation’s sluggish vaccination campaign, the virus remains a moving target. The South African strain hadn’t been detected in this country as of Saturday, according to US officials, but a UK variant has spread to Europe and the United States, including Massachusetts.

Moderna said the South African variant caused a sixfold decrease in disease-fighting antibodies generated by the firm’s two-dose vaccine, which was cleared on Dec. 18 for emergency use by the Food and Drug Administration.


The lower antibody level found in a small study by Moderna and researchers from the National Institutes of Health should still be enough to protect people against the South African strain, Moderna said in a statement, but “may suggest a potential risk of earlier waning of immunity.”

“Out of an abundance of caution,” said Moderna chief executive Stephane Bancel, the company will test a reworked version of the vaccine as a booster shot on laboratory animals and then people “to determine if it will be more effective” against the South African strain and potentially others.

In addition, the company will test whether giving a third shot of the authorized vaccine neutralizes the South African strain.

Moderna’s statement gave no timetable for the new studies. But Bancel told CNBC on Monday that he hoped to test the booster shot tailored to the South African variant “in the clinic very quickly.” He emphasized that the vaccine now being deployed would protect people against the South African strain “in the short term,” and appears to be effective against the UK variant, which is expected to become dominant in the United States by March.


Moderna’s vaccine was the second authorized for emergency use in the United States. It relies on synthetic messenger RNA, the same technology that powers a two-shot vaccine from the partnership of Pfizer and BioNTech, which was cleared a week earlier.

Pfizer said in a statement that the New York-based pharmaceutical giant is “focused on our laboratory studies of the vaccine’s potential” against multiple strains. Dr. Ugur Sahin, the chief executive of Germany-based BioNTech, said in an interview with the New York Times on Monday the company could develop a newly adjusted vaccine against the variants in about six weeks.

The prospect that Moderna’s regimen might ultimately require three shots instead of two would complicate the massive nationwide immunization campaign, which has moved slower than many had hoped.

“If a third shot is needed, it will make the vaccine rollout more difficult,” said Dr. Dan Barouch, who runs the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center and helped design a one-shot COVID-19 vaccine for the health care giant Johnson & Johnson. Results of a late-stage study of that vaccine, which uses different technology, on 45,000 volunteers, including some in South Africa, are expected any day.

Nonetheless, Barouch said it’s premature to read too much into Moderna’s study one way or the other, because it was small. Moderna said it tested how blood samples from eight volunteers in an early-stage trial reacted in the lab to the South African and British strains. Barouch also said scientists are uncertain about exactly what level of antibodies and disease-fighting white blood cells are necessary to protect someone from the coronavirus.


The South African variant was originally detected in October and has been found outside that country. The strain has 10 genetic mutations located on the spike protein that the coronavirus uses to invade cells and has been a key target of vaccine designers.

Officials in Minnesota said on Monday they identified a person infected with a third highly transmissible variant spreading throughout Brazil, according to the Washington Post.

The emergence of new strains comes as no surprise to scientists who study viruses. When a virus replicates itself inside a cell that it has infected, researchers say, the copies often have small differences because of errors in the genetic code, and those errors can lead to mutant strains.

Dr. Anthony Fauci, the nation’s top infectious disease expert, said Monday on the “Today” show that health experts are more concerned about the South African strain than the UK variant. But he appeared to minimize the significance of the decreased antibody level produced by the Moderna vaccine.

“There is a very slight, modest diminution in the efficacy of a vaccine against it, but there’s enough cushion with the vaccines that we have that we still consider them to be effective against both the UK strain and the South African strain,” he said.


The coronavirus variant that first emerged in the United Kingdom in September had no significant impact on the level of disease-fighting antibodies stimulated by Moderna’s vaccine, according to the study. That strain has eight mutations on the spike protein and appears to be more deadly, according to some British scientists.

Messenger RNA vaccines, which had never been approved before the pandemic, use messenger molecules that teach cells to create the spike protein after people are vaccinated. When cells make the protein, it stimulates an immune response that protects people from the virus. Both the Moderna and Pfizer-BioNTech vaccines protected about 95 percent of recipients from the coronavirus in large late-stage studies.

Pfizer and BioNTech said last week that a study by the partnership found that its vaccine worked just as well against the UK strain as it did against the world’s dominant strain. That study, which was published on the preprint server BioRxiv but had yet to be peer-reviewed, didn’t include the South African strain.

Moderna’s study was also published on BioRxiv Monday and had not been subjected to peer review.

Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine and an authority on vaccines, said he wouldn’t be alarmed if a coronavirus vaccine ended up requiring three shots instead of two. Other vaccines have required a three-shot regimen, he said, including those for human papillomavirus, the most common sexually transmitted infection; hepatitis B; and the combination vaccine for diphtheria, tetanus, and whooping cough.


Hotez’s bigger concern, he said, is that the federal government should be sharing findings about the vaccines that it helped bankroll with billions of dollars, not the companies that made them.

“I don’t like hearing my information from company press releases because there’s an inherent bias,” he said. “When it comes to issuing press releases, they’re not doing it for public health concerns. They’re doing it for shareholders. I thought that would be corrected by the Biden administration. So far, it hasn’t been.”

Moderna’s share price rose more than 12 percent to $147 on Monday.

Jonathan Saltzman can be reached at