A girl suffering from measles lies on a bed after being brought to the Mayo Hospital for treatment in Lahore, Pakistan, May 27, 2013. REUTERS/Mohsin Raza

Jodi Krawitt holds her son Rhett, 6, in their home in Corte Madera, California, January 28, 2015. Rhett is recovering from leukemia and his father is concerned his child could succumb to an outbreak of measles at his Northern California school. Krawitt is asking officials to bar entry to any student not vaccinated because of a family's personal beliefs. REUTERS/Elijah Nouvelage

A child cries while receiving a vaccination against measles and rubella at a health center in Sanaa, Yemen, November 11, 2014. REUTERS/Khaled Abdullah

A woman and children wait to receive measles vaccinations operated by M decins Sans �Frontieres at Bangui airport camp, Central African Republic, January 8, 2014. REUTERS/Emmanuel Braun

A student receives a measles vaccination at a primary school in Luoyang, Henan province, China, September 11, 2010. CREUTERS/Carlf Zhang

A girl suffering from measles lies in her mother's lap after being brought to the Mayo Hospital for treatment in Lahore, Pakistan, May 27, 2013. REUTERS/Mohsin Raza

Health workers monitor a neighborhood to apply vaccines against measles in Mexico City, July 22, 2011. REUTERS/Tomas Bravo

A boy reacts while receiving a vaccination against measles and rubella in a school in Sanaa, Yemen, November 11, 2014. REUTERS/Khaled Abdullah

Parents and children line up outside the Paediatric Outpatient department at Morriston Hospital in Swansea, south Wales, following an increase in the number of confirmed measles cases in the region, April 6, 2013. REUTERS/Rebecca Naden

A boy is vaccinated against measles as part of a vaccination program at the Dodowa new town health outreach point in Dodowa, Ghana, April 25, 2012. REUTERS/GAVI/Olivier Asselin

A nurse immunizes a young girl for measles at an International Medical Corps clinic in Gendrassa camp in Maban, Upper Nile State, South Sudan, on the border with Sudan, August 1, 2012. REUTERS/Margaret Aguirre/International Medical Corps

A vaccination point against measles and rubella in a health point in El Alto, in the outskirts of La Paz, Bolivia, April 15, 2012. REUTERS/David Mercado

A child cries when given measles vaccination at Playen district in Gunung Kidul, near the earthquake-hit Indonesian city of Yogyakarta, June 7, 2006. REUTERS/Beawiharta

A girl with measles sits under a tent at Tomping camp in Juba, South Sudan, January 10, 2014. REUTERS/Andreea Campeanu

A girl cries while receiving a vaccination against measles and rubella at a health center in Sanaa, Yemen, November 11, 2014. REUTERS/Khaled Abdullah

A medic attends to a boy suffering from measles, at the Mayo Hospital in Lahore, Pakistan, May 27, 2013. REUTERS/Mohsin Raza

A child cries as he is vaccinated during a mass anti-measles campaign at a health post an hour's drive outside the border town of Moyale in Borena zone, Ethiopia, March 24, 2006. REUTERS/File

Health workers monitor a neighborhood to apply vaccines against measles in Mexico City, July 22, 2011. REUTERS/Tomas Bravo

A boy is given an MMR injection by a qualified school nurse at the Paediatric Outpatients department at Morriston Hospital in Swansea, south Wales, April 6, 2013. REUTERS/Rebecca Naden

A newly-arrived Somali refugee child is held by her mother as she receives a measles vaccine at a refugee camp in Dadaab, near the Kenya-Somalia border, August 1, 2011. REUTERS/Thomas Mukoya