Video report by ITV News Health Editor Emily Morgan
Giving an update from Downing Street on vaccine supply, the prime minister said the UK "remains on track" to see loved ones and reopen hospitality venues, leisure facilities and shops, which is currently carded for April 12 in England.
Mr Johnson cautioned that this will only happen so long as "the data continues to go in the right direction".
Watch the update in full:
He continued that May 17 is the hoped date to allow groups of up to six to mix indoors and an end to restrictions on June 21.
Mr Johnson echoed Health Secretary Matt Hancock who earlier on Thursday said the reduction in available doses of the Oxford/AstraZeneca jab will likely begin on Monday, March 29, and last for around four weeks.
Mr Hancock said the issues were caused by a need to retest 1.7 million doses and a delay in deliveries from the Serum Institute of India (SII) where many of the vaccines are being produced.
Mr Johnson said this was due to the UK's "rigorous safety" programme.
A letter to local health leaders in England, which first exposed the delay, asked vaccination centres and community pharmacy-led services to close unfilled bookings and “ensure no further appointments are uploaded” to booking systems in April.
NHS bosses said that as a result of the supply issues, people under the age of 50 should only get the jab if they are in a priority group, meaning younger adults could face a longer wait to be vaccinated.
However, Mr Johnson said the UK's vaccine targets would be met despite the delays, with all over-50s and clinically vulnerable people offered at least one dose of the vaccine by April 15, with all UK adults offered a first dose by the end of July.
“Let me assure you that if you come forward after receiving your letter, we have the jabs for you," the PM said during the update.
“We’ve always said in a vaccination programme of this pace and scale some interruptions in supply are inevitable and it is true that in the short-term we’re receiving fewer vaccines than we had planned for a week ago.
“That is because of a delay in a shipment from the Serum Institute, who are doing a Herculean job in producing vaccines in such large quantities, and because of a batch that we currently have in the UK that needs to be retested as part of our vigorous safety programme.
“So, as a result, we will receive slightly fewer vaccines in April than in March, but that is still more than we received in February.”
The PM suggested there were no issues with the delivery of Pfizer vaccines, saying: "We will get on and deliver all the second doses" of this jab, when asked about threats by the EU to block vaccine exports.
“I think that people should be under no anxiety or no misapprehensions about that," Mr Johnson said.
“It’s very important to stress, whatever you may hear about the pressures that different countries are under to deliver vaccines for their public, these vaccines are a multi-national effort, they are produced as the result of international co-operation.
“I want to stress that we in the UK will continue to view it in that spirit. We don’t have any bans on exporting stuff and we will continue to co-operate with our European friends.”
Speaking alongside the PM was the head of the UK's medicines regulator, who said there is "no difference" in blood clot levels between people who have had a Covid-19 vaccine and those who have not.
Earlier on Thursday the EU’s vaccine regulator followed the UK's lead and ruled the Oxford/AstraZeneca coronavirus vaccine is safe to use and there is no proven link between the jab and blood clots.
Rollouts in countries such as France and Germany have been paused as a precaution after a patient died due to blood clots after having the vaccine, but there is no evidence linking the condition to any Covid-19 jabs.
Following the ruling by the European Medicines Agency (EMA) a number of countries said they would begin using the AstraZeneca jab again.
EMA Executive Director Emer Cooke said on Thursday that the benefits of the AstraZeneca coronavirus vaccine “outweigh the possible risks”, adding: “This is a safe and effective vaccine.
"Its benefits in protecting people from Covid-19, with the associated risks of death and hospitalisation, outweigh the possible risks.
“The committee also concluded that the vaccine is not associated with an increase in the overall risk of thromboembolic events, or blood clots.”
CEO of the UK's version, the Medicines and Healthcare products Regulatory Association (MHRA), Dr June Raine, said evidence showed there was "no difference" between blood clots occurring in people who were vaccinated and those who had not received the Covid jab.
Mr Johnson opened the briefing by stating there was a "clear scientific conclusion" that the AstraZeneca jab is "safe and effective".
"The thing that isn't safe is catching Covid," he added.
The PM went on to say that he will be receiving a dose of the Oxford/AstraZeneca vaccine on Friday.
Mr Johnson is 56 and currently everyone over the age of 50 in the UK is being asked to book a vaccine appointment.
Dr Raine said the MHRA had received five reports of “a different, a rare form of blood clot" in the brain "occurring together with lowered blood platelets shortly after vaccination with the Covid-19 vaccination AstraZeneca”.
She added: “This type of blood clot can rarely occur naturally in unvaccinated people as well as with people with Covid-19 disease.”
She said a further review of these reports was ongoing but “a causal relationship with the vaccine has not yet been established”.
Dr Raine said the rate of occurrence for CSVT events among the 11 million people vaccinated “is extremely rare”.
She added that while the MHRA continued to investigate reports of blood clots, as a precautionary measure it would advise “anyone with a headache that lasts more than four days after vaccination or bruising beyond the site of vaccination after a few days to seek medical attention”.
Dr Raine urged all those offered the Covid vaccine to take it, saying the benefits “firmly remain to outweigh the risks”.
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