Johnson & Johnson's Covid-19 vaccine candidate should hopefully show high effectiveness of 80% to 85%, the chief adviser for Operation Warp Speed Moncef Slaoui said on Wednesday.
"My expectation is high efficacy," Slaoui said at a J P Morgan healthcare conference, noting that he hoped anything at 80% or above would receive emergency use authorization in the US.
The vaccine, not yet authorized for emergency use in the United States, is in late-stage trials as a single dose vaccine, in contrast to rival two-shot vaccines from Pfizer Inc and Moderna Inc.
Operation Warp Speed is the US government's program to develop and distribute Covid-19 vaccines.
J&J's vaccine is being produced in the United States, Europe, South Africa and India with the help of contract manufacturers in order to build capacity.
It also has the advantage of being a single-shot vaccine, which means it can protect more people faster, and without the cold storage requirements of the other vaccines.
"We are very confident that the vaccine will be much higher than 60%," said the group's scientific director Paul Stoffels.
Interim results from the company's Phase I/II study, published on Wednesday in the New England Journal of Medicine, have helped boost that confidence.
The study showed that 90% of 805 volunteers aged 18 to 55 developed protective antibodies 29 days after a single dose, and that increased to 100% by day 57. The study is ongoing, but the protection has lasted 71 days so far.
Similar data in participants over age 65 will be available in late January.
The study also evaluated the effect of two doses of the vaccine given 56 days apart, and found the booster led to more than double the level in neutralizing antibodies against the virus.
Side effects such as fever, muscle aches and injection site pain, were tolerable and resolved quickly.
Stoffels said the interim data, combined with monkey studies published in the summer showing strong protection against disease and transmission after a single dose, increased his confidence in the vaccine.