A medical staff member fills a syringe with an anti-typhoid vaccine before administering it to a student during the immunisation campaign at a school in Karachi, Pakistan November 20, 2019. REUTERS/Akhtar Soomro

A medical staff member holds a bottle of anti-typhoid vaccine to be given to children during the immunisation campaign at a school in Karachi, Pakistan November 20, 2019. REUTERS/Akhtar Soomro

A student reacts as she gets a free anti-typhoid vaccine during the immunisation campaign at a school in Karachi, Pakistan November 20, 2019. REUTERS/Akhtar Soomro

Non-identical twins Kehinde Akinbola and Taiwo Akinbola snack while waiting to receive their immunisations at the hospital in Igbo Ora, Oyo State, Nigeria April 4, 2019. PIcture taken April 4, 2019. REUTERS/Afolabi Sotunde

Children gather around a girl receiving a polio vaccination during a house-to-house polio immunisation campaign in Sanaa, Yemen November 26, 2018. REUTERS/Mohamed al-Sayaghi

A migrant man, part of a caravan of thousands traveling from Central America en route to the United States, receives vaccination for influenza A H1N1 in a makeshift camp in Mexico City, Mexico, November 15, 2018. REUTERS/Ueslei Marcelino

A World Health Organization (WHO) worker prepares to administer a vaccination during the launch of a campaign aimed at beating an outbreak of Ebola in the port city of Mbandaka, Democratic Republic of Congo May 21, 2018. REUTERS/Kenny Katombe

A World Health Organization (WHO) worker prepares to administer a vaccination during the launch of a campaign aimed at beating an outbreak of Ebola in the port city of Mbandaka, Democratic Republic of Congo May 21, 2018. REUTERS/Kenny Katombe

Doses of influenza A H1N1 vaccines are seen in a cooler at a hospital in Tegucigalpa, Honduras May 15, 2018. REUTERS/Jorge Cabrera

A woman is vaccinated during a campaign of vaccination against yellow fever in Rio de Janeiro, Brazil, March 25, 2017. REUTERS/Ricardo Moraes

Women wait to have their children examined and immunised at a health clinic in the capital Juba, South Sudan, January 23, 2017. Picture taken January 23, 2017. REUTERS/Siegfried Modola

Lead Protein Production and Centrifugation scientist Paul Koelle operates a centrifuge control panel where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Lead Protein Production and Centrifugation scientist Paul Koelle operates a centrifuge control panel where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Research scientist Dan Galperin works on Purified Recombinant Zika Enveloped Protein in his laboratory where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Research scientist Dan Galperin works on Purified Recombinant Zika Enveloped Protein in his laboratory where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Protein Sciences Inc. President and CEO Manon Cox speaks during an interview with Reuters at a Protein Sciences Inc. research laboratory where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Research scientist Dan Galperin works on Purified Recombinant Zika Enveloped Protein at his laboratory where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Research scientist Dan Galperin works on Purified Recombinant Zika Enveloped Protein in his laboratory where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Manufacturing Associate Theodore Szmurlo transfers a cell sample to a microscope slide as he works in the cell culture room where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Research scientist Dan Galperin points to a screen displaying an isolated Purified Recombinant Zika Enveloped Protein in his laboratory where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Manufacturing Associates Theodore Szmurlo (L) and David Artus work in a cell culture room where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Manufacturing Associate Theodore David Artus works in a cell culture room where scientists are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. To match Analysis HEALTH-ZIKA/VACCINES REUTERS/Mike Segar

Manufacturing Associate Theodore Szmurlo views a cell sample with a microscope as he works in the cell culture room where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Manufacturing Associates Theodore Szmurlo and David Artus work in the cell culture room where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Lead Protein Production and Centrifugation scientist Paul Koelle looks into a large bio reactor where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Research scientist Dan Galperin uses a dropper during his work on Purified Recombinant Zika Enveloped Protein at the research laboratory where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

Research scientist Dan Galperin holds vials marked "Zika" during his work on Purified Recombinant Zika Enveloped Protein at the research laboratory where they are working on developing a vaccine for the Zika virus based on production of recombinant variations of the E protein from the Zika virus at the Protein Sciences Inc. headquarters in Meriden, Connecticut, U.S., June 20, 2016. Picture taken June 20, 2016. REUTERS/Mike Segar

An Afghan refugee child returning from Pakistan is vaccinated after arriving at a United Nations High Commissioner for Refugees (UNHCR) registration centre in Kabul, Afghanistan September 27, 2016. Picture taken September 27, 2016. REUTERS/Mohammad Ismail

A girl is seen at a polio immunisation health centre in Maiduguri, Borno State, Nigeria, August 29, 2016. REUTERS/Afolabi Sotunde SEARCH "BAMA" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

A child is given a dose of polio vaccine at an immunisation health centre, in Maiduguri, Borno State, Nigeria, August 29, 2016. REUTERS/Afolabi Sotunde SEARCH "BAMA" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

A woman waits at a polio immunisation health centre, in Maiduguri, Borno State, Nigeria, August 29, 2016. REUTERS/Afolabi Sotunde SEARCH "BAMA" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES.

A South Sudan refugee girl reacts during immunisation and screening by UNICEF health team at the UNHCR managed refugees reception point at Elegu, within Amuru district of the northern region near the South Sudan-Uganda border, August 21, 2016, before being transported to Bidibidi settlement camp. REUTERS/James Akena

A South Sudan refugee girl reacts during immunisation and screening by UNICEF health team at the UNHCR managed refugees reception point at Elegu, within Amuru district of the northern region near the South Sudan-Uganda border, August 21, 2016, before being transported to Bidibidi settlement camp. REUTERS/James Akena

A nurse displays a flu vaccine at a free medical and dental health clinic in Los Angeles, California, U.S., April 27, 2016. REUTERS/Lucy Nicholson

A nurse displays a tetanus, diphtheria and pertussis whooping cough vaccine at a free medical and dental health clinic in Los Angeles, California, U.S., April 27, 2016. REUTERS/Lucy Nicholson

A girl carries a boy during a house-to-house anti-polio vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

A health worker holds a girl to give her polio vaccination drops during a house-to-house anti-polio vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

Girls wait outside their house during a house-to-house anti-polio vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

Children wait to receive polio vaccination drops during a house-to-house vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

A boy reacts as he is about to be given polio vaccination drops during a house-to-house anti-polio vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

A girl waits outside her house during a house-to-house anti-polio vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

A girl cries as she receives polio vaccination drops during a house-to-house vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

A girl receives polio vaccination drops during a house-to-house vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

A boy with differently colored eyes looks on as he waits in the door of his family's house during a house-to-house anti-polio vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

A girl cries as she receives polio vaccination drops during a house-to-house vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

Children wait on a street near a house during a house-to-house anti-polio vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

A boy stands in the door of his house during a house-to-house anti-polio vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

A girl receives polio vaccination drops during a house-to-house vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah TPX IMAGES OF THE DAY

A boy is given vitamin A drops during a house-to-house anti-polio vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah

A boy with differently colored eyes looks on as he waits at the door of his family's house during a house-to-house anti-polio vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah TPX IMAGES OF THE DAY

A girl reacts as she receives polio vaccination drops during a house-to-house vaccination campaign in Yemen's capital Sanaa, April 12, 2016. REUTERS/Khaled Abdullah TPX IMAGES OF THE DAY

A member of the Syrian Arab Red Crescent administers a vaccination to a girl at a school in the rebel held Douma neighborhood of Damascus, Syria March 21, 2016. REUTERS/Bassam Khabieh TPX IMAGES OF THE DAY

Students gesture during a vaccine campaign by the Syrian Arab Red Crescent at a school in the rebel held Douma neighborhood of Damascus, Syria March 21, 2016. REUTERS/Bassam Khabieh

A girl wears a Syrian Arab Red Crescent hat during a vaccine campaign at a school in the rebel held Douma neighborhood of Damascus, Syria March 21, 2016. REUTERS/Bassam Khabieh

Students wear Syrian Arab Red Crescent uniforms during a vaccine campaign at a school in the rebel held Douma neighborhood of Damascus, Syria March 21, 2016. REUTERS/Bassam Khabieh

A boy reacts as a Syrian Arab Red Crescent member administers a vaccination at a school in the rebel held Douma neighborhood of Damascus, Syria March 21, 2016. REUTERS/Bassam Khabieh

Students wear Syrian Arab Red Crescent uniforms during a vaccine campaign at a school in the rebel held Douma neighborhood of Damascus, Syria March 21, 2016. REUTERS/Bassam Khabieh

A boy receives polio vaccine drops outside his family home in Karachi, Pakistan, February 15, 2016. More than 100,000 health workers fanned out across Pakistan on Monday, stepping up a drive to eliminate the polio virus this year from one of its last bastions, despite continuing militant threats to vaccination teams. REUTERS/Akhtar Soomro

A baby girl receives polio vaccine drops at the vaccination center in Karachi, Pakistan, February 15, 2016. More than 100,000 health workers fanned out across Pakistan on Monday, stepping up a drive to eliminate the polio virus this year from one of its last bastions, despite continuing militant threats to vaccination teams. REUTERS/Akhtar Soomro

A girl receives polio vaccine drops at the door step of her family home in Karachi, Pakistan, February 15, 2016. More than 100,000 health workers fanned out across Pakistan on Monday, stepping up a drive to eliminate the polio virus this year from one of its last bastions, despite continuing militant threats to vaccination teams. REUTERS/Akhtar Soomro TPX IMAGES OF THE DAY