The COVID-19 vaccine developed by Pfizer and its German partner, BioNTech, is more than 90% effective at least six months after the second dose is given, the companies announced Thursday.

The vaccine also is effective against the more highly contagious variant prevalent in South Africa, the drugmakers said.

Updated data from a Phase 3 clinical trial of more than 46,000 participants — 12,000 of whom were at least six months out from their second dose — found the vaccine to be 91.3% effective against COVID-19 from seven days up to six months after the second shot. It was 100% effective at preventing severe cases, as defined by the Centers for Disease Control and Prevention. 

Of the 927 participants who had confirmed symptomatic cases of COVID-19, all but 77 were in the placebo group. 

All nine of the cases found in 800 participants in South Africa were from the placebo group, suggesting that the vaccine is highly effective protecting against the variant that was first detected there. According to the CDC, more than 300 cases of that variant have been confirmed in the U.S.

“The high vaccine efficacy observed through up to six months following a second dose and against the variant prevalent in South Africa provides further confidence in our vaccine’s overall effectiveness,” Pfizer Chairman and CEO Albert Bourla said in a news release.

“It is an important step to further confirm the strong efficacy and good safety data we have seen so far, especially in a longer-term follow-up," BioNTech CEO Ugur Sahin added. "These data also provide the first clinical results that a vaccine can effectively protect against currently circulating variants, a critical factor to reach herd immunity and end this pandemic for the global population.”

The companies said the vaccine’s efficacy was generally consistent across age, gender, race and ethnicity demographics, as well as among those with a range of underlying conditions.

No serious safety concerns were observed in trial participants, the vaccine makers said. The most common side effects reported were pain at the injection site, fatigue, headache, muscle pain, chills and joint pain.

The Food and Drug Administration in December granted the Pfizer-BioNTech vaccine emergency use authorization for people ages 16 and older, making it the first vaccine to be administered in the U.S. outside clinical trials. The companies said at the time early data showed the vaccine to be 94.5% effective.

Pfizer and BioNTech announced Wednesday that data shows their vaccine is 100% effective in children 12 to 15 years old and are expected to ask the FDA for emergency use authorization for that age group in the coming weeks.

The CDC said earlier this week that a real-world study found that the Pfizer and Moderna vaccines – both of which use mRNA technology – were found to be 90% effective after two doses and 80% effective two weeks after the initial shot.