BioNTech and its US partner Pfizer have said tests have shown that their Covid-19 vaccine can stand warmer temperatures than initially thought, potentially simplifying the jab's complex cold-chain logistics.

The companies said they have asked the US Food and Drug Administration to allow for the vaccine to be stored for up to two weeks at between -25C to -15C, temperatures commonly found in pharmaceutical freezers and refrigerators.

Under the existing guidelines, the Pfizer-BioNTech jab needs to be stored at between -80C and -60C until five days before use, in a delicate process that requires special ultra-cold containers for shipping and dry ice for storage.

"If approved, this new storage option would offer pharmacies and vaccination centres greater flexibility in how they manage their vaccine supply," said Pfizer CEO Albert Bourla in a statement.

The Pfizer-BioNTech jab, based on novel mRNA technology, was the first vaccine against Covid-19 to be approved in the West late last year.

It was soon followed by US firm Moderna's vaccine, which uses similar technology but can remain stable at -20C for six months and at normal fridge temperature for up to 30 days.

Another approved shot, developed by AstraZeneca and Oxford, uses more traditional vaccine methods and can be stored and shipped at standard fridge temperatures.

BioNTech CEO Ugur Sahin said BioNTech and Pfizer were continuing to work on "new formulations that could make our vaccine even easier to transport and use".

The firms have also started testing their Covid-19 vaccine on healthy pregnant women.

The trial involves some 4,000 pregnant women in the United States, Canada, Argentina, Brazil, Chile, Mozambique, South Africa, Britain and Spain.

Those in the US have already received their first dose, BioNTech and Pfizer said earlier this week.

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Study indicates Pfizer first dose 85% effective after 2-4 weeks

The first dose of the Pfizer-BioNTech vaccination is 85% effective against coronavirus infection between two and four weeks after inoculation, according to a study published in the Lancet medical journal.

The survey was carried out on healthcare workers at the largest hospital in Israel, which on 19 December
launched a mass vaccination campaign regarded as the world's fastest.

Israelis receive the Pfizer-BioNtech vaccine in Hod Hasharon

Israeli studies have found the Pfizer-BioNTech vaccine to be 95% effective one week after a second jab, while the Lancet report focused on more than 9,000 medical staff at Sheba hospital near Tel Aviv.

Some 7,000 of them received the first dose and the rest were not inoculated.

From the group, 170 were diagnosed with Covid-19 after tests carried out only on those showing symptoms or who had been in contact with coronavirus carriers.

52% of them were found to have not been vaccinated.

Comparing the two groups, the Sheba study calculated that the vaccine was 47% effective between one and 14 days after inoculation, rising to 85% after 15 to 28 days.

"What we see is a really high effectiveness already right after two weeks, between two weeks to four weeks after vaccine, already high effectiveness of 85% reduction of symptomatic infection," Gili Regev-Yochay, co-author of the study, told journalists.

He said that despite the vaccine being "amazingly effective", scientists are still studying whether fully vaccinated people can transmit the virus to others.

"That is the big, big, question. We are working on it. This is not on this paper and I hope we will have some good news soon," he said.

Israel has delivered a shot of the Pfizer-BioNTech vaccine to 4.23 million residents, or 47% of its nine million population, 2.85 million of whom have received the recommended full course of two jabs, latest health ministry figures show.

Elsewhere, AstraZeneca and Oxford University's Covid-19 vaccine is more effective when its second dose is given three months after the first, instead of six weeks, a peer-reviewed study published in The Lancet medical journal has shown.

The study confirmed the Anglo-Swedish drugmaker's findings from earlier this month that showed the vaccine had 76% efficacy against symptomatic coronavirus infection for three months after the first dose.

Efficacy was found to be at 81% with the longer interval of 12 weeks between the first and second dose, compared with 55% efficacy up to the six-week gap, according to the Lancet study, which backs British and WHO recommendations for longer intervals. 

Faced with a resurgence in infections and new, highly transmissible variants of the virus, many countries are hoping to broaden immunisation by giving some protection to as many people as possible with a first dose, while delaying subsequent shots.