The United States on Friday authorized Moderna's COVID-19 vaccine for emergency use, paving the way for 6 million doses of a second vaccine to soon begin shipping across the hardest-hit country in the world.
Food and Drug Administration (FDA) chief Stephen Hahn said: "With the availability of two vaccines now for the prevention of COVID-19, the FDA has taken another crucial step in the fight against this global pandemic."
"Congratulations, the Moderna vaccine is now available!" tweeted President Donald Trump.
The US is the first nation to authorize the two-dose regimen from Moderna, now the second vaccine to be deployed in a Western country after the first, developed by Pfizer and BioNTech.
The Pfizer-BioNTech vaccine was approved by Britain on December 2, 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 Thursday it was a "remarkable achievement" to have developed and authorized 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 United States 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 hospitalized, according to the COVID Tracking Project.
Millions of doses will now start shipping out as early as this weekend from cold-storage sites outside Memphis and Louisville, overseen by logistics firm McKesson.
From there they will be delivered to sites around the country via partnerships with FedEx and UPS.
Moderna has several other drugs under development, but has never before seen any authorized.
The decade-old, 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 percent of people against COVID-19 compared to 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 in its monitoring.
Both vaccines come with warnings for people who have histories of allergic reactions to their ingredients to avoid having them.
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, but not enough to attribute causality.
The FDA said immunocompromised people may have diminished immune reaction, and said there was not enough data to say whether there were any particular risks associated to giving the vaccine to pregnant people.
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 -20 degrees Celsius, unlike Pfizer's, which must be stored at -90 degrees Celsius.
Moderna carried out a clinical trial of more than 30,000 people, roughly half of whom 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 criticized 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.
The US has paid for 200 million doses, enough to vaccinate 100 million people.
Moderna proposes to ship 20 million this month and 80 million more in the first quarter of 2021, with the remaining 100 million in the second quarter.
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