The US Health and Human Services Secretary, Alex Azar, has said that the US is likely to have enough safe and effective COVID-19 vaccines available to inoculate the most vulnerable Americans by the end of 2020, Reuters news agency reported on Thursday.

During a news conference, officials were quoted as saying that the US government is "cautiously optimistic" that one or two vaccines, likely from Pfizer Inc or Moderna Inc, will be available by the end of the year and can begin to be distributed to Americans.

Azar said he expects all seniors, healthcare workers and first responders will be able to receive a vaccine as soon as January 2021, with the rest of the American public able to get a vaccine by April 2021.

According to Reuters, companies participating in the US government's effort to develop a vaccine for COVID-19, named Operation Warp Speed, have begun developing manufacturing capabilities even before any vaccinations have been authorised by regulators.