Lead medical assistant Fengmei Lin administers the vaccine for measles, mumps, and rubella (MMR) to one-year-old Bella Huang at the International Community Health Services clinic in Seattle, Washington, U.S., March 20, 2019. Picture taken March 20, 2019. REUTERS/Lindsey Wasson 

Lead medical assistant Fengmei Lin holds a vial of the measles, mumps, and rubella virus (MMR) vaccine at the International Community Health Services clinic in Seattle, Washington, U.S., March 20, 2019. Picture taken March 20, 2019. REUTERS/Lindsey Wasson 

Lead medical assistant Fengmei Lin holds a box of measles, mumps, and rubella (MMR) vaccine vials at the International Community Health Services clinic in Seattle, Washington, U.S., March 20, 2019. Picture taken March 20, 2019. REUTERS/Lindsey Wasson 

A vial of the measles, mumps, and rubella (MMR) vaccine is pictured at the International Community Health Services clinic in Seattle, Washington, U.S., March 20, 2019. Picture taken March 20, 2019. REUTERS/Lindsey Wasson 

A vial of the measles, mumps, and rubella virus (MMR) vaccine is pictured at the International Community Health Services clinic in Seattle, Washington, U.S., March 20, 2019. Picture taken March 20, 2019. REUTERS/Lindsey Wasson 

A vial of the measles, mumps, and rubella (MMR) vaccine is pictured at the International Community Health Services clinic in Seattle, Washington, U.S., March 20, 2019. Picture taken March 20, 2019. REUTERS/Lindsey Wasson 

A vial of measles, mumps and rubella vaccine and an information sheet is seen at Boston Children's Hospital in Boston, Massachusetts February 26, 2015. REUTERS/Brian Snyder (UNITED STATES - Tags: HEALTH) 

A nurse holds a vial of measles, mumps and rubella vaccine at Boston Children's Hospital in Boston, Massachusetts February 26, 2015. REUTERS/Brian Snyder (UNITED STATES - Tags: HEALTH) 

A vial of measles, mumps and rubella vaccine and an information sheet is seen at Boston Children's Hospital in Boston, Massachusetts February 26, 2015. REUTERS/Brian Snyder (UNITED STATES - Tags: HEALTH) 

A vial of measles, mumps and rubella vaccine and an information sheet is seen at Boston Children's Hospital in Boston, Massachusetts February 26, 2015. REUTERS/Brian Snyder (UNITED STATES - Tags: HEALTH) 

A measles vaccine is seen at Venice Family Clinic in Los Angeles, California February 5, 2015. Lawmakers in several U.S. states are backing proposals to make it harder for parents to opt out of school vaccinations based on personal beliefs, as health officials fight a growing measles outbreak that has sickened more than 100 people in more than a dozen states. REUTERS/Lucy Nicholson (UNITED STATES - Tags: HEALTH POLITICS) 

A mother carries her son as they wait for a polio vaccine shot at a health centre in Tegucigalpa April 25, 2012. The Honduran Health Ministry on Wednesday began a nationwide campaign to vaccinate more than 1.4 million children against polio, hepatitis B, meningitis, rotavirus, pneumococcal infection, measles, rubella and mumps, local media reported. REUTERS/Jorge Cabrera (HONDURAS - Tags: HEALTH) 

A nurse shows polio vaccines at a health centre in Tegucigalpa April 25, 2012. The Honduran Health Ministry on Wednesday began a nationwide campaign to vaccinate more than 1.4 million children against polio, hepatitis B, meningitis, rotavirus, pneumococcal infection, measles, rubella and mumps, local media reported. REUTERS/Jorge Cabrera (HONDURAS - Tags: HEALTH) 

Women and their children wait in line to receive a polio vaccine shot at a health centre in Tegucigalpa April 25, 2012. The Honduran Health Ministry on Wednesday began a nationwide campaign to vaccinate more than 1.4 million children against polio, hepatitis B, meningitis, rotavirus, pneumococcal infection, measles, rubella and mumps, local media reported. REUTERS/Jorge Cabrera (HONDURAS - Tags: HEALTH) 

An infant receives a polio vaccine shot at a health centre in Tegucigalpa April 25, 2012. The Honduran Health Ministry on Wednesday began a nationwide campaign to vaccinate more than 1.4 million children against polio, hepatitis B, meningitis, rotavirus, pneumococcal infection, measles, rubella and mumps, local media reported. REUTERS/Jorge Cabrera (HONDURAS - Tags: HEALTH) 

Dr Andrew Wakefield (C) and his wife Carmel (R) receive flowers from a supporter as they leave the General Medical Council in London July 16, 2007. Wakefield, the British doctor who sparked a health scare by suggesting a childhood vaccine against measles, mumps and rubella is linked to autism, faced a hearing on Monday into charges of professional misconduct during his research. REUTERS/Kieran Doherty (BRITAIN) 

Dr Andrew Wakefield (C) is hugged by a supporter as he and his wife Carmel (R) leave the General Medical Council in London July 16, 2007. Wakefield, the British doctor who sparked a health scare by suggesting a childhood vaccine against measles, mumps and rubella is linked to autism, faced a hearing on Monday into charges of professional misconduct during his research. REUTERS/Kieran Doherty (BRITAIN) 

Dr Andrew Wakefield (R) and his wife Carmel leave the General Medical Council in London July 16, 2007. Wakefield, the British doctor who sparked a health scare by suggesting a childhood vaccine against measles, mumps and rubella is linked to autism, faced a hearing on Monday into charges of professional misconduct during his research. REUTERS/Kieran Doherty (BRITAIN) 

Dr Andrew Wakefield leaves the General Medical Council in London July 16, 2007. Wakefield, the British doctor who sparked a health scare by suggesting a childhood vaccine against measles, mumps and rubella is linked to autism, faced a hearing on Monday into charges of professional misconduct during his research. A supporter holds a sign which reads: "Witch Hunt" behind Wakefield. REUTERS/Kieran Doherty (BRITAIN) 

Dr Andrew Wakefield (R) and his wife Carmel leave the General Medical Council in London July 16, 2007. Wakefield, the British doctor who sparked a health scare by suggesting a childhood vaccine against measles, mumps and rubella is linked to autism faced a hearing on Monday into charges of professional misconduct during his research. REUTERS/Kieran Doherty (BRITAIN) 

Dr Andrew Wakefield (L) and his wife Carmel receive flowers as they leave the General Medical Council in London July 16, 2007. Wakefield, the British doctor who sparked a health scare by suggesting a childhood vaccine against measles, mumps and rubella is linked to autism, faced a hearing on Monday into charges of professional misconduct during his research. REUTERS/Kieran Doherty (BRITAIN) 

Palestinian students hold up posters of the Measles, Mumps and Rubella (MMR) vaccination at their school in Mnezel village, south of the West Bank city of Hebron, May 12, 2005. The MMR vaccination aims to protect 1,200,000 Palestinians from the disease in the north of the West Bank, the Palestinian Health Ministry said. 

Palestinian students hold up posters of the Measles, Mumps and Rubella (MMR) vaccination at their school in Mnezel village, south of the West Bank city of Hebron, May 12, 2005. The MMR vaccination aims to protect 1,200,000 Palestinians from the disease in the north of the West Bank, the Palestinian Health Ministry said. REUTERS/Nayef Hashlamoun NH/GOT 

Foreign domestic helpers wear masks to protect themselves from a
mysterious form of pneumonia in Hong Kong March 23, 2003. Experts have
identified the disease as being part of the paramyxovirus family of
viruses that causes measles and mumps. REUTERS/Bobby Yip

KC/RCS 

Britain's Chief Medical Officer Sir Liam Donaldson (R) speaks at a news
conference with Professor Sian Griffiths - President of the Faculty of
Public Health Medicine at the Department of Health, February 7, 2002 in
London. Britain's Prime Minister Tony Blair is coming under increasing
pressure to say whether his baby son Leo has been given the Measles,
Mumps and Rubella vaccine (MMR) amid growing fears a drop in the number
of children having the jab might spark a measles epidemic.
REUTERS/Stephen Hird

PS 

Britain's Chief Medical Officer Sir Liam Donaldson speaks at a news
conference at the Department of Health February 7, 2002 in London.
Britain's Prime Minister Tony Blair is coming under increasing pressure
to say whether his baby son Leo has been given the Measles, Mumps and
Rubella vaccine (MMR) amid growing fears a drop in the number of
children having the jab might spark a measles epidemic. REUTERS/Stephen
Hird

PS 

Britain's Chief Medical Officer Sir Liam Donaldson speaks at a news
conference at the Department of Health February 7, 2002 in London.
Britain's Prime Minister Tony Blair is coming under increasing pressure
to say whether his baby son Leo has been given the Measles, Mumps and
Rubella vaccine (MMR) amid growing fears a drop in the number of
children having the jab might spark a measles epidemic. REUTERS/Stephen
Hird

PS/WS 

Health Ministry worker Virginia Jimienez vaccinates a Costa Rica child against measles and mumps, in the poor neighborhood of La Carpio, north of San Jose, October 21. The Health Ministry is starting a vaccination campaign against measles and mumps that will cover more than 300,000 Costa Rican children.

COSTA RICA-HEALTH