Pfizer vaccine has 91% efficacy for up to six months, trial shows

Findings based on two doses three weeks apart are first to show shot remains effective for many months

The coronavirus vaccine developed by Pfizer and its German partner BioNTech protects against symptomatic Covid for up to six months, an updated analysis of clinical trial data has found.

In a statement released on Thursday, the companies reported efficacy of 91.3% against any symptoms of the disease in participants assessed up to six months after their second shot. The level of protection is only marginally lower than the 95% achieved soon after vaccination.

The findings are the first to demonstrate that the vaccine remains effective for many months, an outcome that doctors and scientists had desperately hoped for because it suggests that people being vaccinated now should be protected at least until the autumn when boosters may be ready.

Analysis of participants in the phase 3 trial, which has enrolled 46,307 people, identified 927 symptomatic Covid cases. Of these, 850 were in the placebo arm of the trial and 77 in the vaccine group. There were 32 cases of severe Covid, as defined by the US Centers for Disease Control and Prevention (CDC), in the placebo group, and none in the vaccinated group. More than 12,000 people vaccinated in the trial have now been followed for at least six months after their second dose.

More striking still are results from the South African arm of the trial where nine Covid cases were observed among 800 participants. All of the cases were in the placebo group, and six were confirmed to be the new “variant of concern”, B.1.351, which has worried scientists because of its ability to partially evade antibodies produced in response to vaccines or past infection. Public Health England said on Wednesday that it knew of 469 confirmed or probable cases of the South African variant in the UK.

With cases in the South African arm of the trial so low, more evidence is needed to confirm the vaccine’s protection against the new variant, but scientists were still delighted by the result. “I do regard this as a really positive indication,” said Danny Altmann, a professor of immunology at Imperial College London.

He said scientists had become “terribly worried” about the variant’s ability to evade immunity from previous infection or vaccination. “Studies like this confirm our sense that the vaccine gives such massive protective headroom that even with some loss of immunity, you’re still safe,” he said.

The chairman of Pfizer, Albert Bourla, said the latest data put the company in a position to apply to the US Food and Drugs Administration for full approval of the vaccine. The jab is currently approved under emergency use authorisation.

The results came as scientists in the UK reported strong immune responsesin older people who had received two shots of the Pfizer vaccine. Blood tests on 100 people aged 80 to 96 years old revealed that 98% produced strong antibody responses after two doses of the vaccine given three weeks apart. Antibody levels more than tripled after the second shot.

The findings, released in a preprint that has yet to be peer-reviewed, will boost confidence that the Pfizer vaccine can be highly effective against Covid even in the most vulnerable older people, who tend to generate far weaker immune responses to vaccines and natural infections. Pfizer trialled its vaccine with a three-week gap between shots, but the UK leaves a three-month gap, meaning it is unclear whether the same level of protection is achieved.

Paul Moss, a professor of haematology at the University of Birmingham who led the study with Dr Helen Parry, also at Birmingham, said the team was surprised and very pleased to see the results, which tallied with the “excellent clinical protection” the Pfizer vaccine appears to provide. The first major real-world study of the Pfizer vaccine, in Israel, found that two shots prevented 94% of symptomatic cases across all age groups.

The scientists went on to examine another branch of the immune defences raised by the vaccine, known as the T-cell response. Antibodies protect against infection by gumming up the virus and preventing it from infecting cells, but T-cells destroy human cells that are already infected, and may also support antibody production over time. After both shots of the vaccine, two-thirds of the participants had detectable T-cell responses. “We know that as people age their cellular immune responses are more difficult to elicit,” said Moss. “So that is something that we will need to keep an eye on very closely.”

Further work at Public Health England’s Porton Down lab showed that blood serum taken from the volunteers after two shots of vaccine strongly neutralised the original coronavirus that spread around the world last year. But it was on average 14 times less effective against the P.1 variant first seen in Brazil, and which has now reached the UK and elsewhere.

“The variant from Brazil reduces neutralisation response, but at this early stage after the vaccine where we’re seeing such high antibody levels we are still quietly confident that this should still provide valuable protection against this variant of concern,” said Parry.