An approved coronavirus vaccine could end up being effective only 50 to 60 per cent of the time, meaning public health measures will still be needed to keep the pandemic under control.
Director of the National Institute of Allergy and Infectious Diseases in USA, also a top infectious diseases expert, Dr Anthony Fauci, came up with the statement on Friday, reports Reuters.
"We don't know yet what the efficacy might be. We don't know if it will be 50 per cent or 60 per cent. I'd like it to be 75 per cent or more," Fauci said in a webinar hosted by Brown University.
"But the chances of it being 98% effective are not great, which means you must never abandon the public health approach."
The novel coronavirus has infected nearly 5 million people in the United States and killed more than 160,000.
Lockdown measures imposed to keep the virus from spreading have devastated the economy, which suffered its biggest blow since the Great Depression in the second quarter, with gross domestic product dropping at its steepest pace in at least 73 years.
As infections have spiked around the country after states started to open up, public health experts, including Fauci, have stressed the importance of steps that each American can take, including social distancing, washing hands and wearing masks.
Fauci said on Friday that studies of Moderna Inc's COVID-19 vaccine could produce definitive data in November or December of this year.
He said earlier this week that he expects tens of millions of COVID-19 vaccine doses to be available by early 2021, and a billion doses by the end of that year.
US President Donald Trump has put forward a more optimistic forecast, saying on Thursday the country could have a coronavirus vaccine before the Nov 3 election.
