Before any vaccine is permitted in the USA, it must be reviewed by the Food and Drug Administration, which requires study in thousands of people.
"It was only made possible because of this President, who said at the same time, "I am pursuing a vaccine; I am going to do something novel". However, the USA government expects to approve the vaccine in the first half of December. How long does protection last?
The trial involved 41,000 people worldwide.
Placebo group get vaccine? . If obtained, as many as 20 million Americans could be immunized by the end of the year, per United States officials. The emergency submission will potentially enable use of the vaccine in high-risk populations in the U.S. by the middle to end of December, and Pfizer will be ready to distribute the vaccine within hours of authorization, the companies said.
But the speed at which all the vaccines have been developed - the vaccine development timeline has been squeezed from an average of 10 years to less than one - have led some to worry about how safe the vaccines will be.
"We know Moderna is 94.5 per cent effective and Pfizer 95 per cent", McEnany said.
Another government group - advisers to the Centers for Disease Control and Prevention - decides who is first in line for scarce doses. Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, said both the FDA and the European Medicines Agency (EMA) would "conduct a very careful evaluation".
The Health Secretary said the NHS has already been organising to proceed with vaccination as soon as the jab obtains authorisation from the MHRA.
Pfizer and BioNTech announced earlier this week that the shot was safe and 95 percent effective in a late-stage trial with almost 44,000 participants. That's a milestone FDA set because historically, vaccine side effects don't crop up later than that.
Typically, a vaccine will inject dead virus cells into the bloodstream, which allows the immune system to recognize the individual virus and produce antibodies that can create an individual's immunity to the virus.
The US this week passed 250,000 deaths in the coronavirus outbreak, by far the largest number in the world.
Japan was also on "maximum alert" after logging a record 2,000 daily infections with almost 500 in the capital Tokyo alone, though no immediate restrictions were planned.
The CDCP has issued a "strong recommendation" that Americans refrain from travelling during the Thanksgiving holiday.
USA states and cities are imposing a raft of new restrictions, including home confinement, the closure of indoor dining and a limit on gatherings as cases soar.
The transition of administrations has also not helped, with President-elect Joe Biden complaining of a lack of co-operation from Donald Trump's government. A new study published in Health Affairs indicates that there are many other factors to consider before the completion of vaccine trials can be celebrated as victories. In October, the Emergency Care Research Institute warned that releasing a vaccine without six months of follow-up data on clinical trial participants could pose "significant safety risks". It too uses the mRNA approach. It took 15 years to develop a vaccine for human papilloma virus, which can cause six kinds of cancer. Later, finalized results showed effectiveness to be closer to 95 percent. Pfizer has been manufacturing doses even while safety and efficacy tests were ongoing, in order to avoid delays in shipping once authorized. The Sinovac and Oxford-AstraZeneca vaccines are also showing potential in their respective trials. The Pfizer and Moderna vaccines have been the only ones to report any results from late-stage trials.