The “best immunity” from the new Covid-19 vaccine occurs from seven days after the second dose, experts said.
People who receive the Pfizer/BioNTech vaccine will receive two doses three weeks apart.
Those who are vaccinated will receive some level of protection around 12 days after the first jab.
But the best protection comes a week after the second dose is delivered.
Professor Sir Munir Pirmohamed, chair of Commission on Human Medicine expert working group, told a Downing Street press conference: “From the data that has been presented to us, people will be immune seven days after the second dose.
“Partial immunity does occur after the first dose. And we can see some protection occurring after day 12 of the first dose, but the best immunity is seven days after the second dose.”
Sir Munir said approval of the vaccine was a “historic moment”.
He told a Government press conference: “We are in the midst of a once-in-a-century pandemic and I think this is a historic moment.
“The UK is now one step closer to providing a safe and effective vaccine to help in the fight against Covid-19, a virus that has affected each and every one of us in some way.
“This will help to save lives.”
He said that regulators had been given “unprecedented access” to the raw data, including clinical trials and manufacturing processes.
“From this we can come to the conclusion that there is overwhelming benefit for this particular vaccine and therefore recommended to the MHRA that its use should be authorised,” he told a No 10 briefing.
“The data showed that this vaccine is 95% effective.
“It is effective within all the groups who were given the vaccine in the trial irrespective of age, sex, race or country they lived in.”
On the safety of the vaccine, he said: “The trials had about 40,000 individuals and half of those received the vaccine and half received the placebo.
“From the data that has been provided most of the adverse effects were mild and short-lasting. Usually lasted for a day or two – similar to the kind of effects you get after any other vaccine.
“So that is really important to note that we haven’t identified any serious adverse reactions throughout the trials.”
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