The company announced Wednesday that preliminary data from last-stage trials had shown it to be 79.3% effective.It is an inactive vaccine, which means the virus was grown in a lab and then killed.

To date, more than 600,000 people across the United Kingdom have already received the Pfizer-BioNTech jab, including Margaret Keenan from Northern Ireland.

Vaccine doses have been distributed to 142 hospitals, health care systems, federally qualified health centers and pharmacies in Pennsylvania, with another 58 locations expected to receive doses this week, the Department of Health said in a news release. "This vaccine will be made available to some of the poorest regions of the world at a low cost, helping protect countless people from this very bad disease", Matt Hancock, U.K. Secretary of State for health and social care, said in a release.

The vaccine is a viral-vector vaccine and can be stored in a standard refrigerator, unlike other vaccines that require cold storage. "But at the same time we have to temper the excitement".

Slaoui reportedly said the phase 3 trial recruitment for the vaccine has been completed and that the company is now working with the Operation Warp Speed team to accelerate the availability of the vaccine doses.

Governments need to secure widespread public acceptance of a new vaccine in order to achieve so-called "herd immunity", but have to contend with anti-vaccine campaigns able to spread their messages rapidly through social media.

After British health officials disclosed the new details about the efficacy of the vaccine, an AstraZeneca spokesman said: "It is for the regulators to authorise how the vaccine should be used".

However, Pfizer said that any "alternative" dosing methods should be tracked by health officials. "The second dose completes the course and is important for longer term protection".

However, questions remain about how well the vaccine protects older people. Only 12% of study participants were over 55 and they were enrolled later, so there hasn't been enough time to see whether they develop infections at a lower rate than those not given the vaccine.

Most coronavirus vaccines to be used in poorer countries likely will be made by the Serum Institute of India, which has been contracted by AstraZeneca to make 1 billion doses.

Beyond Britain, there are hopes the AstraZeneca shot will offer a path out of the pandemic. But none can be provided until approved by the WHO.

Hours later, Argentina became the second nation to approve the vaccine, and was soon followed by El Salvador. Instead, the WHO waits until a health regulator has approved them. Chile is also interested.Britain has set itself apart from other Western countries with its fast-track approach to vaccinations, having green-lighted the Pfizer/BioNTech vaccine weeks before the EU's European Medicines Authority (EMA) did so.

The Associated Press reported this story. Mario Ritter, Jr. was the editor.