Britain's Deputy Chief Medical Officer Jonathan Van-Tam (seen on the screen), Chair of the Joint Committee on Vaccination and Immunisation (JCVI) Professor Wei Shen Lim and Chief Executive of the Medicines and Healthcare Products Regulatory Agency (MHRA) June Raine hold a media conference on coronavirus disease (COVID-19), at Downing Street in London, Britain, November 15, 2021. Frank Augstein/Pool via REUTERS 

Chair of the Joint Committee on Vaccination and Immunisation (JCVI) Professor Wei Shen Lim speaks during a media conference on coronavirus disease (COVID-19), at Downing Street in London, Britain, November 15, 2021. Frank Augstein/Pool via REUTERS 

Britain's Deputy Chief Medical Officer Jonathan Van-Tam (seen on the screen), Chair of the Joint Committee on Vaccination and Immunisation (JCVI) Professor Wei Shen Lim and Chief Executive of the Medicines and Healthcare Products Regulatory Agency (MHRA) June Raine hold a media conference on coronavirus disease (COVID-19), at Downing Street in London, Britain, November 15, 2021. Frank Augstein/Pool via REUTERS 

Chair of the Joint Committee on Vaccination and Immunisation (JCVI) Professor Wei Shen Lim, Britain's Deputy Chief Medical Officer Professor Jonathan Van-Tam, and Chief Executive of the Medicines and Healthcare Products Regulatory Agency (MHRA) June Raine, attend a media briefing on COVID-19 at Downing Street in London, Britain September 14, 2021. Justin Tallis/Pool via REUTERS 

Members of the media listen as Chair of the Joint Committee on Vaccination and Immunisation (JCVI) Professor Wei Shen Lim, Britain's Deputy Chief Medical Officer Professor Jonathan Van-Tam, and Chief Executive of the Medicines and Healthcare Products Regulatory Agency (MHRA) June Raine, hold a news conference on COVID-19 at Downing Street in London, Britain September 14, 2021. Justin Tallis/Pool via REUTERS 

Chair of the Joint Committee on Vaccination and Immunisation (JCVI) Professor Wei Shen Lim attends a media briefing on COVID-19 at Downing Street in London, Britain September 14, 2021. Justin Tallis/Pool via REUTERS 

Chair of the Joint Committee on Vaccination and Immunisation (JCVI) Professor Wei Shen Lim, Britain's Deputy Chief Medical Officer Professor Jonathan Van-Tam, and Chief Executive of the Medicines and Healthcare Products Regulatory Agency (MHRA) June Raine, attend a media briefing on COVID-19 at Downing Street in London, Britain September 14, 2021. Justin Tallis/Pool via REUTERS 

Chair of the Joint Committee on Vaccination and Immunisation (JCVI) Professor Wei Shen Lim attends a media briefing on COVID-19 at Downing Street in London, Britain September 14, 2021. Justin Tallis/Pool via REUTERS 

Chair of the Joint Committee on Vaccination and Immunisation (JCVI) Professor Wei Shen Lim, Britain's Deputy Chief Medical Officer Professor Jonathan Van-Tam, and Chief Executive of the Medicines and Healthcare Products Regulatory Agency (MHRA) June Raine, attend a media briefing on COVID-19 at Downing Street in London, Britain September 14, 2021. Justin Tallis/Pool via REUTERS 

Chair of the Joint Committee on Vaccination and Immunisation (JCVI) Professor Wei Shen Lim, Britain's Deputy Chief Medical Officer Professor Jonathan Van-Tam, and Chief Executive of the Medicines and Healthcare Products Regulatory Agency (MHRA) June Raine, attend a media briefing on COVID-19 at Downing Street in London, Britain September 14, 2021. Justin Tallis/Pool via REUTERS 

Chief Medical Officer Chris Whitty, Doctor June Raine Chief Executive of the Medicines and Healthcare products Regulatory Agency and Professor Wei Shen Lim, chairman of the Joint Committee on Vaccination and Immunisation hold a news conference at 10 Downing Street, amid the coronavirus disease (COVID-19) outbreak, in London, Britain, September 13, 2021. REUTERS/Hannah McKay/Pool 

Chief Medical Officer Chris Whitty, Doctor June Raine Chief Executive of the Medicines and Healthcare products Regulatory Agency and Professor Wei Shen Lim, chairman of the Joint Committee on Vaccination and Immunisation hold a news conference at 10 Downing Street, amid the coronavirus disease (COVID-19) outbreak, in London, Britain, September 13, 2021. REUTERS/Hannah McKay/Pool 

Professor Wei Shen Lim, chairman of the Joint Committee on Vaccination and Immunisation holds a news conference at 10 Downing Street, amid the coronavirus disease (COVID-19) outbreak, in London, Britain, September 13, 2021. REUTERS/Hannah McKay/Pool 

Chief Medical Officer Chris Whitty, Doctor June Raine Chief Executive of the Medicines and Healthcare products Regulatory Agency and Professor Wei Shen Lim, chairman of the Joint Committee on Vaccination and Immunisation hold a news conference at 10 Downing Street, amid the coronavirus disease (COVID-19) outbreak, in London, Britain, September 13, 2021. REUTERS/Hannah McKay/Pool 

Professor Wei Shen Lim, chairman of the Joint Committee on Vaccination and Immunisation holds a news conference at 10 Downing Street, amid the coronavirus disease (COVID-19) outbreak, in London, Britain, September 13, 2021. REUTERS/Hannah McKay/Pool 

Chief Medical Officer Chris Whitty, Doctor June Raine Chief Executive of the Medicines and Healthcare products Regulatory Agency and Professor Wei Shen Lim, chairman of the Joint Committee on Vaccination and Immunisation hold a news conference at 10 Downing Street, amid the coronavirus disease (COVID-19) outbreak, in London, Britain, September 13, 2021. REUTERS/Hannah McKay/Pool 

Chief Medical Officer Chris Whitty, Doctor June Raine Chief Executive of the Medicines and Healthcare products Regulatory Agency and Professor Wei Shen Lim, chairman of the Joint Committee on Vaccination and Immunisation hold a news conference at 10 Downing Street, amid the coronavirus disease (COVID-19) outbreak, in London, Britain, September 13, 2021. REUTERS/Hannah McKay/Pool 

Chief Medical Officer Chris Whitty, Doctor June Raine Chief Executive of the Medicines and Healthcare products Regulatory Agency and Professor Wei Shen Lim, chairman of the Joint Committee on Vaccination and Immunisation holds a news conference at 10 Downing Street, amid the coronavirus disease (COVID-19) outbreak, in London, Britain, September 13, 2021. REUTERS/Hannah McKay/Pool 

Peru's President Pedro Castillo greets a teacher before she receives a dose of the Sinopharm vaccine against the coronavirus disease (COVID-19) during the launch of a mass immunisation rollout for teachers in Lima, Peru, September 3, 2021. REUTERS/Sebastian Castaneda 

Peru's President Pedro Castillo observers a teacher as she receives a dose of the Sinopharm vaccine against the coronavirus disease (COVID-19) during the launch of a mass immunisation rollout for teachers in Lima, Peru, September 3, 2021. REUTERS/Sebastian Castaneda 

Peru's President Pedro Castillo observes a teacher as she receives a dose of the Sinopharm vaccine against the coronavirus disease (COVID-19) during the launch of a mass immunisation rollout for teachers in Lima, Peru, September 3, 2021. REUTERS/Sebastian Castaneda 

A nurse immuniser prepares to administer the AstraZeneca vaccine to a patient at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club during a lockdown to curb an outbreak of cases in Sydney, Australia, August 25, 2021. REUTERS/Loren Elliott 

A nurse immuniser administers the AstraZeneca vaccine to a patient at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club during a lockdown to curb an outbreak of cases in Sydney, Australia, August 25, 2021. REUTERS/Loren Elliott 

A nurse immuniser administers the AstraZeneca vaccine to a patient at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club during a lockdown to curb an outbreak of cases in Sydney, Australia, August 25, 2021. REUTERS/Loren Elliott 

A nurse immuniser prepares to administer the AstraZeneca vaccine to a patient at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club during a lockdown to curb an outbreak of cases in Sydney, Australia, August 25, 2021. REUTERS/Loren Elliott 

Nurse immuniser Kelie Lee (C) prepares to administer the AstraZeneca vaccine to a patient at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club during a lockdown to curb an outbreak of cases in Sydney, Australia, August 25, 2021. REUTERS/Loren Elliott 

Nurse immunisers administer the AstraZeneca vaccine to people at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club during a lockdown to curb an outbreak of cases in Sydney, Australia, August 25, 2021. REUTERS/Loren Elliott 

Nurse immuniser Kelie Lee administers the AstraZeneca vaccine to a patient at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club during a lockdown to curb an outbreak of cases in Sydney, Australia, August 25, 2021. REUTERS/Loren Elliott 

Nurse immuniser Kelie Lee administers the AstraZeneca vaccine to a patient at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club during a lockdown to curb an outbreak of cases in Sydney, Australia, August 25, 2021. REUTERS/Loren Elliott 

Nurse immuniser Kelie Lee prepares to administer the AstraZeneca vaccine to a patient at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club during a lockdown to curb an outbreak of cases in Sydney, Australia, August 25, 2021. REUTERS/Loren Elliott 

Professor Wei Shen Lim, chair of the Joint Committee on Vaccination and Immunisation (JCVI), and Professor Jonathan Van-Tam, Deputy Chief Medical Officer for England give a media briefing on COVID-19 in Downing Street, London, Britain, August 4, 2021. Yui Mok/Pool via REUTERS 

Professor Wei Shen Lim, chair of the Joint Committee on Vaccination and Immunisation (JCVI), Professor Jonathan Van-Tam, Deputy Chief Medical Officer for England, and Dr June Raine, Chief Executive of the Medicines and Healthcare products Regulatory Agency (MHRA), give a media briefing on COVID-19 in Downing Street, London, Britain, August 4, 2021. Yui Mok/Pool via REUTERS 

Professor Wei Shen Lim, chair of the Joint Committee on Vaccination and Immunisation (JCVI), gives a media briefing on COVID-19 in Downing Street, London, Britain, August 4, 2021. Yui Mok/Pool via REUTERS 

Professor Wei Shen Lim, chair of the Joint Committee on Vaccination and Immunisation (JCVI), speaks during a media briefing on COVID-19 in Downing Street, London, Britain, August 4, 2021. Yui Mok/Pool via REUTERS 

Professor Wei Shen Lim, chair of the Joint Committee on Vaccination and Immunisation (JCVI), Professor Jonathan Van-Tam, Deputy Chief Medical Officer for England, and Dr June Raine, Chief Executive of the Medicines and Healthcare products Regulatory Agency (MHRA), give a media briefing on COVID-19 in Downing Street, London, Britain, August 4, 2021. Yui Mok/Pool via REUTERS 

Nurse immuniser Jude Mendoza prepares patient Phiyona Castillo to receive the AstraZeneca vaccine at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club as the city experiences an extended lockdown, in Sydney, Australia, August 3, 2021. REUTERS/Loren Elliott 

Patient Phiyona Castillo receives the AstraZeneca vaccine administered by a nurse immuniser at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club as the city experiences an extended lockdown, in Sydney, Australia, August 3, 2021. REUTERS/Loren Elliott 

A patient receives the AstraZeneca vaccine administered by a nurse immuniser at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club as the city experiences an extended lockdown, in Sydney, Australia, August 3, 2021. REUTERS/Loren Elliott 

A patient receives the AstraZeneca vaccine administered by a nurse immuniser at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club as the city experiences an extended lockdown, in Sydney, Australia, August 3, 2021. REUTERS/Loren Elliott 

Patient Chutikan Duangsri receives the AstraZeneca vaccine administered by nurse immuniser Jude Mendoza at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club as the city experiences an extended lockdown, in Sydney, Australia, August 3, 2021. REUTERS/Loren Elliott 

Nurse immuniser Selena De Leon speaks with patient Karen Brooks after administering the AstraZeneca vaccine at a coronavirus disease (COVID-19) vaccination clinic at the Bankstown Sports Club as the city experiences an extended lockdown, in Sydney, Australia, August 3, 2021. REUTERS/Loren Elliott 

Britain's COVID-19 Vaccine Deployment Minister Nadhim Zahawi, Head of Immunisation for Public Health England Dr Mary Ramsay and medical director of primary care for NHS England Dr Nikki Kanani hold a media briefing on the coronavirus disease (COVID-19) pandemic, at Downing Street in London, Britain, June 23, 2021. REUTERS/Tom Nicholson/Pool 

Head of Immunisation for Public Health England Dr Mary Ramsay speaks during a media briefing on the coronavirus disease (COVID-19) pandemic, at Downing Street in London, Britain, June 23, 2021. REUTERS/Tom Nicholson/Pool 

Britain's COVID-19 Vaccine Deployment Minister Nadhim Zahawi, Head of Immunisation for Public Health England Dr Mary Ramsay and medical director of primary care for NHS England Dr Nikki Kanani hold a media briefing on the coronavirus disease (COVID-19) pandemic, at Downing Street in London, Britain, June 23, 2021. REUTERS/Tom Nicholson/Pool 

Head of Immunisation for Public Health England Dr Mary Ramsay looks at Britain's COVID-19 Vaccine Deployment Minister Nadhim Zahawi as he speaks during a media briefing on the coronavirus disease (COVID-19) pandemic, at Downing Street in London, Britain, June 23, 2021. REUTERS/Tom Nicholson/Pool 

Head of Immunisation for Public Health England Dr Mary Ramsay speaks during a media briefing on the coronavirus disease (COVID-19) pandemic, at Downing Street in London, Britain, June 23, 2021. REUTERS/Tom Nicholson/Pool 

Head of Immunisation for Public Health England Dr Mary Ramsay speaks during a media briefing on the coronavirus disease (COVID-19) pandemic, at Downing Street in London, Britain, June 23, 2021. REUTERS/Tom Nicholson/Pool 

Britain's COVID-19 Vaccine Deployment Minister Nadhim Zahawi, Head of Immunisation for Public Health England Dr Mary Ramsay and medical director of primary care for NHS England Dr Nikki Kanani hold a media briefing on the coronavirus disease (COVID-19) pandemic, at Downing Street in London, Britain, June 23, 2021. REUTERS/Tom Nicholson/Pool 

Britain's COVID-19 Vaccine Deployment Minister Nadhim Zahawi, Head of Immunisation for Public Health England Dr Mary Ramsay and medical director of primary care for NHS England Dr Nikki Kanani hold a media briefing on the coronavirus disease (COVID-19) pandemic, at Downing Street in London, Britain, June 23, 2021. REUTERS/Tom Nicholson/Pool 

Hungary's Prime Minister Viktor Orban shows a Hungarian immunisation certificate against the coronavirus disease (COVID-19) as he answers the media questions, following the meeting with Britain's Prime Minister Boris Johnson, at Downing Street in London, Britain May 28, 2021. REUTERS/Toby Melville 

A medical worker fills a syringe with a dose of AstraZeneca's coronavirus disease (COVID-19) vaccine at the Belexpo vaccination centre, as Serbia started to immunise business people across the region, in Belgrade, Serbia, March 27, 2021. REUTERS/Zorana Jevtic 

Britain's Health Secretary Matt Hancock holds a news conference with Head of Immunisation for Public Health England Mary Ramsay and Deputy Chief Medical Officer for England Jonathan Van-Tam at 10 Downing Street, amid the coronavirus disease (COVID-19) outbreak, in London, Britain March 17, 2021. REUTERS/Hannah McKay/Pool 

Britain's Head of Immunisation for Public Health England Mary Ramsay speaks during a news conference at 10 Downing Street, amid the coronavirus disease (COVID-19) outbreak, in London, Britain March 17, 2021. REUTERS/Hannah McKay/Pool 

Britain's Health Secretary Matt Hancock holds a news conference with Head of Immunisation for Public Health England Mary Ramsay and Deputy Chief Medical Officer for England Jonathan Van-Tam at 10 Downing Street, amid the coronavirus disease (COVID-19) outbreak, in London, Britain March 17, 2021. REUTERS/Hannah McKay/Pool 

Brazil's President Jair Bolsonaro and Brazil's Health Minister Eduardo Pazuello arrive to a ceremony to announce mass immunisation programme against the coronavirus disease (COVID-19), at the Planalto Palace, in Brasilia, Brazil December 16, 2020. REUTERS/Ueslei Marcelino 

Brazil's President Jair Bolsonaro walks after a ceremony to announce mass immunisation programme against the coronavirus disease (COVID-19), at the Planalto Palace, in Brasilia, Brazil December 16, 2020. REUTERS/Ueslei Marcelino 

Brazil's President Jair Bolsonaro greets "Ze Gotinha", the mascot of SUS (Unique Health System), during a ceremony to announce mass immunisation programme against the coronavirus disease (COVID-19), at the Planalto Palace, in Brasilia, Brazil December 16, 2020. REUTERS/Ueslei Marcelino 

Margaret Keenan, 90, is the first patient in Britain to receive the Pfizer/BioNtech COVID-19 vaccine at University Hospital, administered by nurse May Parsons, at the start of the largest ever immunisation programme in the British history, in Coventry, Britain December 8, 2020. Britain is the first country in the world to start vaccinating people with the Pfizer/BioNTech vaccine. Jacob King/Pool via REUTERS/File Photo TPX IMAGES OF THE DAY SEARCH "GLOBAL POY" FOR THIS STORY. SEARCH "REUTERS POY" FOR ALL BEST OF 2020 PACKAGES. 

Grace Thomson receives receives the Pfizer/BioNTech COVID-19 vaccine from Paula McMahon, on the first day of the largest immunisation programme in the British history, at the Louisa Jordan Hospital, in Glasgow, Scotland, Britain December 8, 2020. Jeff J Mitchell/Pool via REUTERS 

Kay Gallwey Chand, 84, receives the Pfizer/BioNTech COVID-19 vaccine from a senior nurse Dilhani Somaweera, on the first day of the largest immunisation programme in the British history, at the Royal Free Hospital, in London, Britain December 8, 2020. Jack Hill/Pool via REUTERS