Suzanne Spencer is already vaccinated against COVID-19, one of the first to get Moderna’s two-shot regime during clinical trials in Seattle last spring.

But the 76-year-old Mercer Island woman rolled up her sleeve again this week to help test an experimental booster shot designed to protect against the most worrisome new variant of the novel coronavirus.

“This is the virulent one, the nasty one,” said Spencer, a retired family physician.

She and two other Seattle-area residents got the shot Thursday, helping kick off an early-stage trial of a vaccine tailored for B.1.351, a variant that was detected in South Africa late last year and is now spreading around the globe. At least 17 cases have been detected in Washington since January, according to the state health department.

The trial will evaluate vaccine safety and immune responses in more than 200 volunteers. That will include about 50 people in Seattle who, like Spencer, are already vaccinated against the original version of the virus.

“We’re investigating possible strategies for use of a vaccine like this,” said study co-leader Dr. Lisa Jackson of Kaiser Permanente Washington Health Research Institute.

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The shot could be used exclusively as a booster, for example, or as a stand-alone vaccine. Different doses are also being investigated, along with mixtures of the two formulations.

A total of about 150 people who have not yet been vaccinated will be enrolled in Seattle and at the three other sites where the trial is being conducted: Nashville, Atlanta and Cincinnati.

Jackson was leader of the first Moderna trial in Seattle — the world’s first human trial of a COVID-19 vaccine. Researchers are continuing to track those volunteers, Jackson said.

With the emergence of new variants, vaccine makers have begun tweaking their formulations — something that’s particularly easy to do with messenger RNA vaccines like Moderna’s.

The new vaccine is essentially identical to the one being widely administered, except that it includes the mRNA code for the variant’s spike protein, which the virus uses to attach to and infect cells.

“It’s a subtle difference,” Jackson said.

Of all the variants that have emerged over the past several months, none is causing as much anxiety as B.1.351. Not only does it appear to be about 50% more infectious than the original strain, but it is also the most adept at evading both natural and vaccine-induced immunity.

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The changes are due to mutations in the spike protein that give the variant an advantage.

Existing vaccines still appear to provide “an adequate level of protection” against all circulating variants, Dr. Anthony Fauci, director of the National Institutes of Allergy and Infectious Diseases said in a statement announcing the trial, which NIAID is funding.

“However, out of an abundance of caution, NIAID has continued its partnership with Moderna to evaluate this variant vaccine candidate should there be a need for an updated vaccine,” Fauci said.

Nationally, the CDC reports 323 infections caused by the B.1.351 variant. In South Africa, the variant spread so rapidly it accounted for more than 90 percent of infections within a month of its discovery, Jackson pointed out.

“We’re kind of in the midst of an explosion of variants that are more infectious and have a real advantage in terms of being able to spread in the population,” she said. “Until we can get a handle on transmission, we are setting up a breeding ground for variants.”

The Food and Drug Administration said in February that vaccines modified to fight the variants won’t have to go through the same lengthy human trials as the original versions, but it’s still not clear exact what the streamlined approval process will include, Jackson added.

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After her first two Moderna shots, Spencer didn’t experience any side effects. Thursday’s shot left her a little achy, but by Friday she was fine.

It’s a small price to pay for being able to help with such important research, she said. “As a physician, I loved my patients and I hated retiring,” she said. “This is a way of giving back.”

Volunteers can register for the trial at: https://corona.kpwashingtonresearch.org/.