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

Josephine Faleye 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, in London, Britain December 8, 2020. Jack Hill/Pool via REUTERS

Roger Shaw, 87, receives a COVID-19 vaccination record card from a nurse Naomi Walsh, on the first day of the largest immunisation programme in the British history, in London, Britain December 8, 2020. Jack Hill/Pool via REUTERS

A COVID-19 vaccination record card is held at a vaccination centre, on the first day of the largest immunisation programme in the British history, in London, Britain December 8, 2020. Jack Hill/Pool via REUTERS

A man receives the Pfizer/BioNTech COVID-19 vaccine at Croydon's University Hospital, on the first day of the largest immunisation programme in the British history, in London, Britain December 8, 2020. Dan Charity/Pool via REUTERS

George Dyer, 90, arrives to receive the first Pfizer/BioNTech COVID-19 vaccine at Croydon's University Hospital, on the first day of the largest immunisation programme in the British history, in London, Britain December 8, 2020. Dan Charity/Pool via REUTERS

George Dyer, 90, receives the first Pfizer/BioNTech COVID-19 vaccine, administered by General manager of Covid Recovery Becky Board, at Croydon's University Hospital, on the first day of the largest immunisation programme in the British history, in London, Britain December 8, 2020. Dan Charity/Pool via REUTERS

George Dyer, 90, receives the first Pfizer/BioNTech COVID-19 vaccine, administered by General manager of Covid Recovery Becky Board, at Croydon's University Hospital, on the first day of the largest immunisation programme in the British history, in London, Britain December 8, 2020. Dan Charity/Pool via REUTERS

Dr Nicola Steedman interim Deputy Chief Medical Officer conducts a tv interview, as the first of two Pfizer/BioNTech COVID-19 vaccine jabs are being given to the staff, at the Western General Hospital, on the first day of the largest immunisation programme in the British history, in Edinburgh, Scotland Britain December 8, 2020. Andrew Milligan/Pool via REUTERS

NHS staff look on as the first of two Pfizer/BioNTech COVID-19 vaccine jabs are being administered to staff, at the Western General Hospital, on the first day of the largest immunisation programme in the British history, in Edinburgh, Scotland Britain December 8, 2020. Andrew Milligan/Pool via REUTERS

Dr Nicola Steedman interim Deputy Chief Medical Officer conducts a tv interview, as the first of two Pfizer/BioNTech COVID-19 vaccine jabs are being given to the staff, at the Western General Hospital, on the first day of the largest immunisation programme in the British history, in Edinburgh, Scotland Britain December 8, 2020. Andrew Milligan/Pool via REUTERS

Leaflets sit on a table , as the first of two Pfizer/BioNTech COVID-19 vaccine jabs are being given to the staff, at the Western General Hospital, on the first day of the largest immunisation programme in the British history, in Edinburgh, Scotland Britain December 8, 2020. Andrew Milligan/Pool via REUTERS

Deputy charge nurse Katie McIntosh waits to administer the first of two Pfizer/BioNTech COVID-19 vaccine jabs to the staff, at the Western General Hospital, on the first day of the largest immunisation programme in the British history, in Edinburgh, Scotland Britain December 8, 2020. Andrew Milligan/Pool via REUTERS

A woman receives the first of two Pfizer/BioNTech COVID-19 vaccine jabs, at Guy's Hospital, at the start of the largest ever immunisation programme in the British history, in London, Britain December 8, 2020. Victoria Jones/Pool via REUTERS

A phial of the Pfizer/BioNTech COVID-19 vaccine concentrate is diluted with 1.8ml sodium chloride ready for use at Guy's Hospital, at the start of the largest ever immunisation programme in the British history, in London, Britain December 8, 2020. Victoria Jones/Pool via REUTERS

A box of phials of the Pfizer/BioNTech COVID-19 vaccine concentrate is transported from storage ready for use at Guy's Hospital, at the start of the largest ever immunisation programme in the British history, in London, Britain December 8, 2020. Victoria Jones/Pool via REUTERS

A man receives the first of two Pfizer/BioNTech COVID-19 vaccine jabs, at Guy's Hospital, at the start of the largest ever immunisation programme in the British history, in London, Britain December 8, 2020. Victoria Jones/Pool via REUTERS

Frances Gogh receives the first of two Pfizer/BioNTech COVID-19 vaccine jabs, at Guy's Hospital, at the start of the largest ever immunisation programme in the British history, in London, Britain December 8, 2020. Victoria Jones/Pool via REUTERS

Dr Doreen Brown, 85, receives the first of two Pfizer/BioNTech COVID-19 vaccine jabs, at Guy's Hospital, at the start of the largest ever immunisation programme in the British history, in London, Britain December 8, 2020. Victoria Jones/Pool via REUTERS

A man receives the first of two Pfizer/BioNTech COVID-19 vaccine jabs, at Guy's Hospital, at the start of the largest ever immunisation programme in the British history, in London, Britain December 8, 2020. Victoria Jones/Pool via REUTERS

Vivien McKay Clinical Nurse Manager raises her hands in the air after Deputy charge nurse Katie McIntosh(C) administered the first of two Pfizer/BioNTech COVID-19 vaccine jabs, watched by Dr Nicola Steedman interim Deputy Chief Medical Officer at the Western General Hospital, on the first day of the largest immunisation programme in the British history, in Edinburgh, Scotland Britain December 8, 2020. Andrew Milligan/Pool via REUTERS

Deputy charge nurse Katie McIntosh administers the first of two Pfizer/BioNTech COVID-19 vaccine jabs, to Clinical Lead of Outpatient Theatres Andrew Mencnarowski, at the Western General Hospital, on the first day of the largest immunisation programme in the British history, in Edinburgh, Scotland Britain December 8, 2020. Andrew Milligan/Pool via REUTERS

Deputy charge nurse Katie McIntosh administers the first of two Pfizer/BioNTech COVID-19 vaccine jabs, to Vivien McKay Clinical Nurse Manager at the Western General Hospital, on the first day of the largest immunisation programme in the British history, in Edinburgh, Scotland Britain December 8, 2020. Andrew Milligan/Pool via REUTERS

Deputy charge nurse Katie McIntosh shows leaflets to Clinical Lead of Outpatient Theatres Andrew Mencnarowski after she administered the first of two Pfizer/BioNTech COVID-19 vaccine jabs, the Western General Hospital, on the first day of the largest immunisation programme in the British history, in Edinburgh, Scotland Britain December 8, 2020. Andrew Milligan/Pool via REUTERS

Deputy charge nurse Katie McIntosh administers the first of two Pfizer/BioNTech COVID-19 vaccine jabs, to Vivien McKay Clinical Nurse Manager at the Western General Hospital, on the first day of the largest immunisation programme in the British history, in Edinburgh, Scotland Britain December 8, 2020. Andrew Milligan/Pool via REUTERS

A nurse administers the Pfizer/BioNTech COVID-19 vaccine to a man at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

A nurse administers the Pfizer/BioNTech COVID-19 vaccine to a woman at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

A nurse administers the Pfizer/BioNTech COVID-19 vaccine to a man at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

A nurse administers the Pfizer/BioNTech COVID-19 vaccine to a woman at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

A nurse administers the Pfizer/BioNTech COVID-19 vaccine to a woman at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

Medical personnel prepare to administer the Pfizer/BioNTech COVID-19 vaccine to recipients at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

A nurse administers the Pfizer/BioNTech COVID-19 vaccine to a woman at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

A nurse administers the Pfizer/BioNTech COVID-19 vaccine to a woman at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

A nurse speaks to a man prior to administering the Pfizer/BioNTech COVID-19 vaccine at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

Medical personnel wait to administer the Pfizer/BioNTech COVID-19 vaccine at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

Medical personnel are briefed prior to administering the Pfizer/BioNTech COVID-19 vaccine at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

Medical personnel wait to administer the Pfizer/BioNTech COVID-19 vaccine at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

Medical personnel prepare to administer the Pfizer/BioNTech COVID-19 vaccine at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

People wait before and after receiving the Pfizer/BioNTech COVID-19 vaccine at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS TPX IMAGES OF THE DAY

People rest after receiving the Pfizer/BioNTech COVID-19 vaccine at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

Medical personnel prepare to administer the Pfizer/BioNTech COVID-19 vaccine at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Justin Tallis/Pool via REUTERS

A member of medical staff checks phials of the Pfizer/BioNTech COVID-19 vaccine in a refrigerator at University Hospital, on the first day of the largest immunisation programme in the British history, in Coventry, Britain December 8, 2020. Jacob King/Pool via REUTERS

A nurse holds a phial of the Pfizer/BioNTech COVID-19 vaccine at University Hospital, on the first day of the largest immunisation programme in the British history, in Coventry, Britain December 8, 2020. Jacob King/Pool via REUTERS

A nurse holds a phial of the Pfizer/BioNTech COVID-19 vaccine at University Hospital, on the first day of the largest immunisation programme in the British history, in Coventry, Britain December 8, 2020. Jacob King/Pool via REUTERS

A needle next to a phial of the Pfizer/BioNTech COVID-19 vaccine jab is seen at the Royal Victoria Infirmary, on the first day of the largest immunisation programme in the British history, in Newcastle, Britain December 8, 2020. Owen Humphreys/Pool via REUTERS

A nurse prepares to administer the Pfizer/BioNTech COVID-19 vaccine jab at the Royal Victoria Infirmary, on the first day of the largest immunisation programme in the British history, in Newcastle, Britain December 8, 2020. Owen Humphreys/Pool via REUTERS

A member of staff prepares a phial of the Pfizer/BioNTech COVID-19 vaccine jab ahead of it being administered at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Ben Birchall/Pool via REUTERS

Two members of medical staff check the Pfizer/BioNTech COVID-19 vaccine jab ahead of it being administered at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Ben Birchall/Pool via REUTERS

A woman receives the first of two Pfizer/BioNTech COVID-19 vaccine jabs at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Ben Birchall/Pool via REUTERS

A phial of the Pfizer/BioNTech COVID-19 vaccine jab is seen after being administered at a vaccination centre, on the first day of the largest immunisation programme in the British history, in Cardiff, Wales, Britain December 8, 2020. Ben Birchall/Pool via REUTERS