Pfizer's COVID-19 shot remains highly protective against the coronavirus, longer-term data shows

• Pfizer released new data from a study indicating lasting benefits for its COVID-19 vaccine.
• The company said the two-dose vaccine was 91% effective at preventing symptomatic cases of COVID-19.
• Data from study volunteers in South Africa suggests the shot also repels concerning variants.

Updated results from Pfizer's late-stage study of its coronavirus vaccine indicate that the two-dose shot's overwhelming protection is holding up six months after the second dose.

The study has tallied 927 symptomatic cases of COVID-19 among the more than 46,000 trial volunteers. People who got placebo shots accounted for 850 of those cases, compared with 77 illnesses among those who received the vaccine, which was developed by Pfizer and BioNTech. That translates to 91.3% efficacy, the companies said in a statement Thursday.

Additionally, the companies said the shot was 95% effective at preventing severe COVID-19. Twenty-one cases of severe disease, as defined by the US Food and Drug Administration, were seen in the placebo group, compared with one case in the vaccinated group.

The end result is unambiguous good news: More than 12,000 participants have been followed for at least six months, and protection is still holding strong. Pfizer's release provides the first clinical data showing vaccine efficacy endures for at least six months and counting.

It's still unknown when protection may start to wane, leaving an open question of whether people may need to get booster shots.

Pfizer and BioNTech also said they didn't identify any safety concerns with the vaccine after following trial participants for a longer period of time.

The companies first put out data in November showing the shot was highly effective against COVID-19, and they won emergency authorization for the vaccine in the US in December. Almost 77 million doses of the shot have since been given in the US, according to data from the Centers for Disease Control and Prevention.

The trial also provides the first clinical data suggesting the Pfizer-BioNTech vaccine works against B.1.351, a variant first identified in South Africa that has caused some concern among virologists. This is a "critical factor to reach herd immunity," Ugur Sahin, BioNTech's CEO, said in a statement.

Petri-dish testing showed leading COVID-19 vaccines produced less of an immune response against this variant, and the COVID-19 vaccine from Oxford University and AstraZeneca faltered against it in a human trial.

Pfizer's study enrolled 800 participants in South Africa, and nine of them came down with COVID-19. All nine of the cases happened in the placebo group. Researchers sequenced those strains and found six of the nine were the B.1.351 strain.

Pfizer CEO Albert Bourla said in a statement that the extended follow-up time and efficacy against the B.1.351 variant "provides further confidence in our vaccine's overall effectiveness."

Pfizer shared the updated analysis in a press release. These newest results have yet to be published in a peer-reviewed medical journal.