Adults in the United States who were vaccinated against measles decades ago may need a new dose depending on when they received the shot and their exposure risk, according to public health experts battling the nation’s largest outbreak since the virus was deemed eliminated in 2000.
Up to 10 percent of the 695 confirmed measles cases in the current outbreak occurred in people who received one or two doses of the vaccine, according to the U.S. Centers for Disease Control and Prevention.
The figure illustrates what can happen when a large number of individuals, even those who have been vaccinated, are exposed to the measles. CDC recommends that people who are living in or traveling to outbreak areas should check their vaccination status and consider getting a new dose.
Dr. Allison Bartlett, an infectious disease expert at the University of Chicago Medicine, said the “continued vulnerability to infection” is why high-risk adults such as healthcare workers are routinely advised to get a second dose of the measles vaccine if they have not had one.
But knowing your vaccination status can be tricky, experts said.
“It’s complicated and often futile because it's very difficult to resurrect those old records,” said Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center.
People vaccinated in the United States since 1989 would most likely have received two doses of the combined measles, mumps and rubella (MMR) shot under federal guidelines, and that is still considered the standard for protection.
Anyone vaccinated between 1963 and 1989 would likely have received only one dose, with many people immunized in the earlier years receiving an inactivated version of the virus. Americans born before 1957 are considered immune as they would have been exposed to the virus directly in an outbreak.
Merck & Co Inc is the sole U.S. provider of the MMR vaccine. The company said in a statement that it has “taken steps to increase U.S. supply” of the vaccine due to the current outbreak.
The measles virus is highly contagious and can cause blindness, deafness, brain damage or death. It is currently spreading in outbreaks in many parts of the world.
According to the World Health Organization, 95 percent of a population needs to be vaccinated to provide “herd immunity,” a form of indirect protection that prevents infection in people too young or sick to be vaccinated. U.S. public health officials have blamed the current outbreak in part on rising rates of vaccine skepticism that have reduced measles immunity in certain communities.
For travelers to outbreak areas abroad, the CDC recommends adults consider getting another dose of MMR unless they have proof of receiving two prior doses, take a blood test showing immunity, or were born before 1957.
In general, the CDC says two doses of the measles vaccine should provide 97 percent protection; one dose should offer 93 percent protection. However, immunity can wane over time.
This has occurred even in adults with two documented doses of the vaccine, said Dr. Michael Phillips, chief epidemiologist at NYU Langone Health, which serves parts of New York City, a hot spot in the U.S. outbreak.
He said in kids, “the vaccine is really effective,” but in some adults, memory T-cells, which recognize and attack germs, do not fight the virus as effectively as they once did.
Rapid blood tests are available that can detect whether a person is immune based on the level of measles antibodies, but the tests are not 100 percent reliable.
Adults who have any doubt about their immunity should get another dose, Schaffner said: "It's safe. There's no downside risk. Just roll up your sleeve.”
Treatment, preventative care and vaccinations for measles across the world
Treatment, preventative care and vaccinations for measles across the world
A child receives a vaccination against measles by a family physician on April 16, 2018 in the Romanian capital, Bucharest.
Measles still claims young lives in Romania, where nearly 40 children have died in an outbreak that many blame on parents being misled by scare stories that vaccinating them is dangerous. Some 12,000 people have contracted measles since late 2016 in the European Union's second-poorest country, 46 of them died. Among the dead, 39 were children under the age of three who had not been vaccinated, making Romania one of the worst affected countries in the ongoing measles outbreak in Europe. / AFP PHOTO / Daniel MIHAILESCU (Photo credit should read DANIEL MIHAILESCU/AFP/Getty Images)
A family physician prepares a measles vaccine during a consultation on April 16, 2018 in the Romanian capital, Bucharest. - Measles still claims young lives in Romania, where nearly 40 children have died in an outbreak that many blame on parents being misled by scare stories that vaccinating them is dangerous. Some 12,000 people have contracted measles since late 2016 in the European Union's second-poorest country, 46 of them died. Among the dead, 39 were children under the age of three who had not been vaccinated, making Romania one of the worst affected countries in the ongoing measles outbreak in Europe. (Photo by Daniel MIHAILESCU / AFP) (Photo credit should read DANIEL MIHAILESCU/AFP/Getty Images)
A nurse was seen vaccinating school children at her school in Lhokseumawe, August 4, 2018, Aceh, Indonesia. Vaccination Measles-Rubella (MR) for Indonesian school children is part of the Indonesian government's health program to provide physical immunity from two dangerous diseases, Rubella and Measles in Indonesian children. (Photo by Fachrul Reza/NurPhoto via Getty Images)
Artists wearing superhero costumes comfort an elementary student while she receives a German Measles, also known as Rubella, vaccination in Pekanbaru, Riau province on August 1, 2018 as part of the Indonesian governments health program to combat childhood diseases. (Photo by WAHYUDI / AFP) (Photo credit should read WAHYUDI/AFP/Getty Images)
ALEPPO, SYRIA - APRIL 17 : A Syrian child is vaccinated during the measles campaign in Al-Bab district of Aleppo, Syria on April 17, 2018. Kids fled from Eastern Ghouta's Douma after Assad regime forces' suspected chemical attack on April 7, 2018 receive measles vaccine Al-Bab district of Aleppo. (Photo by Omer Alven/Anadolu Agency/Getty Images)
A Venezuelan woman is vaccinated against measles in Cucuta, Colombia, at the international brigde Simon Bolivar on the border with Venezuela, on March 21, 2018. / AFP PHOTO / Schneyder Mendoza (Photo credit should read SCHNEYDER MENDOZA/AFP/Getty Images)
Venezuelan, Yan Manuel, receives a free measles vaccine given by a volunteer, after showing his identity card at the Pacaraima border control, Roraima State, Brazil August 19, 2018. REUTERS/Nacho Doce
A member of the Free Revolutionary Police carries out an inspection on a measles vaccine in the southern Idlib countryside September 17, 2014. Fifteen children died after being vaccinated against measles in northern Syria, resulting in the programme being halted, aid workers said on Wednesday, a tragedy likely to damage trust in health services in opposition-held areas. The Free Revolutionary Police is a newly launched unarmed police movement that works in areas controlled by the Free Syrian Army to solve the day-to-day problems in Idlib, the group said. Picture taken September 17, 2014. REUTERS/Khalil Ashawi (SYRIA - Tags: CIVIL UNREST CONFLICT HEALTH)
A girl receives anti-measles vaccination drops at a health centre in BASECO compound in Tondo, Manila September 3, 2014. Philippine President Benigno Aquino said on September 1, 2014 between 11 to 13 million people in the country are at risk from measles, polio and rubella (German measles), and asked the public to cooperate in eradicating the preventable diseases, during Monday's launch of a mass national vaccination campaign against measles and polio, reported local media. REUTERS/Erik De Castro (PHILIPPINES - Tags: HEALTH SOCIETY)
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)
Rowan Walters, aged four, is held by her mother as she given an MMR injection at the Paediatric Outpatients department at Morriston Hospital in Swansea, south Wales April 6, 2013. Following an increase in the number of confirmed cases of a measles in south Wales, parents in the area were urged to vaccinate their children, and hospitals in the region opened drop-in clinics on Saturday, local media reported. REUTERS/Rebecca Naden (BRITAIN - Tags: SOCIETY HEALTH)
A health worker in Ciudad Juarez at the Mexican border crossing with El Paso, Texas, hands out leaflets to passing motorists and people as part of a measles prevention programme, February 16, 2015. California public health officials have confirmed three more cases of measles in an outbreak that began in late December, bringing to 113 the total number of people believed to have been infected in the state. Health officials in Arizona, where seven cases of measles have been documented, said the outbreak would likely be considered over in that state if no further infections were reported over the weekend. Across the United States, more than 150 people have been diagnosed with measles, many of them linked to an outbreak that authorities believe began when an infected person from out of the country visited Disneyland in late December. REUTERS/Jose Luis Gonzalez (MEXICO - Tags: SOCIETY HEALTH IMMIGRATION)