The United States has authorised a second Covid-19 vaccine for emergency use against the pandemic, that is killing more than 2,500 people a day in the US.
The Food and Drug Administration (FDA) is the world’s first health regulator to approve the two-dose Moderna jab, around one week after giving approval to the vaccine developed by Pfizer and BioNTech, which is now being distributed.
Following the announcement, President Donald Trump tweeted: "Congratulations, the Moderna vaccine is now available!"
Congratulations, the Moderna vaccine is now available!— Donald J. Trump (@realDonaldTrump) December 19, 2020
The Pfizer-BioNTech vaccine was first approved by the UK on 2 December and was followed by several other countries, including the US, last week.
Less-vetted shots have also been rolled out in China and Russia.
Meharry Medical College President James Hildreth, who was part of a panel of experts convened by the FDA to discuss approval matters, said it was a "remarkable achievement" to have developed and authorised the Pfizer and Moderna vaccines within the space of a year.
Together, they offer a glimmer of light at the end of the pandemic's long tunnel.
The US alone has seen more than 310,000 people die from coronavirus infections and is in the midst of a brutal winter surge, with nearly 115,000 people hospitalised, according to the Covid Tracking Project.
Millions of doses will start being distributed as soon as this weekend from cold-storage sites in Tennessee and Kentucky state, overseen by logistics firm McKesson.
From there they will be delivered to sites around the US via partnerships with FedEx and UPS.
Moderna has several other drugs under development, but has never before seen any authorised.
The Massachusetts-based biotech company received $2.5 billion in federal funding for its efforts and co-developed the vaccine with scientists at the National Institutes of Health.
Both the Pfizer and Moderna vaccines are based on cutting-edge mRNA (messenger ribonucleic acid) technology, and both have been shown to be highly effective, protecting about 95% of people against Covid-19 compared with a placebo.
They have also been found to have no serious safety issues in clinical trials involving tens of thousands of people each.
The most commonly reported side effects were pain at the injection site, tiredness, headache, muscle pain, chills, joint pain, swollen lymph nodes in the same arm as the injection, nausea and vomiting, and fever.
But there have now been a handful of people around the world who developed significant allergic reactions after receiving the Pfizer vaccine, and the FDA has said it will remain vigilant.
Both vaccines come with warnings for people who have histories of allergic reactions to their ingredients.
FDA scientist Peter Marks told reporters that the total number of cases of allergic reactions to the Pfizer vaccine across the United States was now "roughly five".
He said investigations were under way, but said the culprit might be an ingredient called polyethylene glycol, which is in both vaccines.
The FDA will also be on the lookout to see whether both vaccines may in extremely rare cases be linked to Bell's palsy, a relatively mild and temporary facial paralysis condition.
There were a handful of such cases in the clinical trials.
Both of the vaccines work by giving human cells the instructions to make a surface protein of the coronavirus, which simulates an infection and trains the immune system for when it encounters the real virus.
They each differ in the formulation of the fatty particles used to deliver the mRNA, which allows Moderna's vaccine to be kept in long term storage at -20C (-4F), unlike Pfizer's, which must be stored at -70C.
Moderna carried out a clinical trial of more than 30,000 people, roughly half of who were given the product and the other half a placebo, with neither recipients nor their health care providers knowing who was in each group.
It has proposed to "unblind" the whole study and offer placebo recipients the vaccine, a plan criticised by some experts who say scientists will be deprived of valuable data, and some people will end up getting the vaccine ahead of others in their priority group.
Moderna proposes to distribute 20m this month and 80m more in the first quarter of 2021, with the remaining 100m in the second quarter.