The effectiveness of the Pfizer vaccine as a shield against coronavirus has been found to wane slightly over time, but remains strongly protective for at least six months after the second dose.
That finding, released by the company on Wednesday, may be considered by US health authorities in deciding if and when booster doses might be needed.
Pfizer and its German partner BioNTech have said they plan to seek authorisation for boosters.
The new data comes from the 44,000-person study that initiated the widespread use of the vaccine, showing it to be highly effective in the first few months after immunisation. Now the companies have tracked those study participants for six months and counting.
Most importantly, protection against severe COVID-19 remains very high, at nearly 97 per cent, researchers found. Overall, protection against symptomatic COVID-19 was 91 per cent over the six-month period.
But efficacy against any symptomatic infection dropped gradually every two months, from a peak of 96 per cent two months after the study participants had their second dose. By month four, efficacy was 90 per cent and by six months, it was about 84 per cent.
The study results were posted online but haven't undergone full scientific review.
They don't single out how the vaccine works against the highly contagious delta variant. However, the companies cite separate testing and real-world data showing the shots do counter that strain.
Australian Associated Press