Britain has secured access to 90 million doses of two "promising" coronavirus vaccines, the government said Friday, taking its total orders of potential Covid-19 vaccines to nearly 350 million doses.

The country has struck agreements to buy 60 million doses from US biotech firm Novavax, and another 30 million doses from Janssen pharmaceuticals, which is part of US giant Johnson & Johnson.

The orders follow previous deals reached with four other companies, including BioNTech, Pfizer and Valneva, and mean Britain has agreed access to four different vaccine types being produced by six firms.

"Since we don't know if any of these vaccines are going to work, we think it's important to have a spread of risk across the different types," Kate Bingham, Head of the UK Vaccines Task Force, told AFP.

"We do need a broad portfolio of different vaccines because I don't think there will be a single vaccine that will be suitable for everybody."

The new agreements include co-funding a "ground-breaking global clinical study" of the Janssen vaccine, the government said.

Meanwhile Novavax will conduct the key third phase of its vaccine trials with the support of Britain's National Institute for Health Research, it added.

In its single biggest order to date, the UK has said it will purchase 100 million doses of a vaccine currently being trialed by Oxford University in partnership with AstraZeneca.

Bingham said she expected to strike further deals with other pharma companies, adding the most optimistic timeline for any to begin delivering doses was by the end of the year.

"This is a good start but will there be better and different vaccines coming in the future," she said. "This virus is here to stay."

Countries around the world have been racing to strike agreements for some of the dozens of Covid-19 vaccines currently in development. 

Russia said this week it had developed the world's first vaccine offering "sustainable immunity" against the virus and was in the final stage of human trials.

Vietnam's health ministry is looking to buy up to 150 million doses of the Russian vaccine, state media said on Friday, despite global scepticism over its effectiveness and safety.

Bingham said no Western regulator would currently approve the Russian vaccine given the level of data available.

"There is no reason to think the vaccine doesn't work or is not safe, but there is not data suggesting it's safe or effective," she added.