The effectiveness of the Pfizer vaccine wanes slightly over time but it remains strongly protective for at least six months after the second dose, according to company data.
The findings are one piece of evidence that US health authorities will consider 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 a 44,000-person study that initiated the widespread use of the vaccine, showing it is 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%, researchers found. Overall, protection against symptomatic Covid-19 was 91% over the six-month period.
A closer look shows that efficacy against any symptomatic infection dropped gradually every two months, from a peak of 96% two months after study participants got their second dose.
By month four efficacy was 90%, and by six months it was about 84%.
The study results were posted online but have not undergone full scientific review.
They do not single out how the vaccine works against the highly contagious Delta variant, but the companies cite separate testing and real-world data showing the jabs counter it.
