Moderna announced on Monday that its two-dose COVID-19 vaccine provides strong immunity against two worrisome new coronavirus variants, but the company is developing an additional “booster” shot just in case a South African strain causes that immunity to fade unexpectedly.

The Cambridge, Mass-based biotech firm said the move was “out of an abundance of caution ... against this and potentially future variants.”

The news comes just days after Pfizer-BioNTech released data showing its vaccine protects against a more contagious variant that emerged in Britain and is now showing up in many countries, including the U.S.

Experts say this latest chapter in the pandemic heightens the dire urgency of global vaccination campaigns — as if the world needed any more incentive to rush the shots into arms.

Scott E. Hensley, a University of Pennsylvania microbiologist who is researching the coronavirus, explained that the only way to prevent the emergence of mutant strains is to stop the rampant spread of the coronavirus through vaccination.

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“If we could use the vaccines to reduce the level of virus being transmitted, then we wouldn’t have to worry so much about new variants,” he said. “What I’m afraid of is that people will hear about this and say, ‘The vaccines are losing effectiveness so I’m not going to get vaccinated.’ ”

All viruses constantly mutate as they replicate within infected cells. Most mutations are inconsequential or even detrimental to the microbe, but a year into the pandemic, the coronavirus has found a few that give it a survival advantage.

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The British variant is more transmissible than the original virus that emerged a year ago in Wuhan, China, because it is more efficient at breaking into human airway cells. Data are beginning to suggest the British version is also more harmful.

The South African strain and a similar one that emerged independently in Brazil are potentially more threatening. Not only do they attach to airway cells efficiently, but they also block the action of certain disease-fighting antibodies that are activated by the new vaccines.

“It’s a one-two punch,” Hensley said. “It’s more transmissible, and more” difficult to fight.

Further complicating the fight, Hensley said, is the fact that people who get infected and recover naturally mount a relatively weak antibody defense compared to the ultrahigh levels of antibodies activated by the Pfizer and Moderna vaccines. In Brazil, many people who were infected with the original COVID-19 virus were not immune to reinfection with the new strain.

This suggests that the only way to extinguish the pandemic is to protect a high percentage of the population through vaccination, not through natural immunity.

Moderna conducted lab tests using blood samples from eight people and two monkeys who received both doses of the company’s vaccine. Although the immunization triggered high, protective levels of antibodies against the British and South African variants, the response to the South African variant was significantly weaker.

“These lower [antibody] titers may suggest a potential risk of earlier waning of immunity” to the South African strain, the Moderna news release said.

Moderna’s inoculation and one made by Pfizer-BioNTech received authorization for emergency use from the U.S. Food and Drug Administration last month. Since then, states have been frantically, and somewhat chaotically, rolling out the immunizations while the companies try to ramp up production to meet demand.

Both Moderna and Pfizer used mRNA technology, a novel genetic approach to activating the immune system. The technology enables fast development and production of vaccine candidates; Moderna’s now-authorized version took just 42 days to devise.

A new version could be made “hopefully a little faster this time, but not much,” Moderna’s chief medical officer Tal Zaks told the New York Times.

Whether a booster shot is actually necessary is under study. Antibodies are just one arm of the immune system. The authorized vaccines also generate T cells and B cells, which “remember” an invader and guard against reinfection.