I have been vaccinated for measles. Am I protected against new strains that have infected 11 people in Ontario?
Reader Sarah Polk in Montreal writes,”Does the immunity that Canadians currently have due to measles mumps and rubella (MMR) vaccination protect us against new strains that have infected 11 people in Ontario?” The Globe’s public health reporter André Picard says, “The short answer is yes.” While there are various strains of the measles, Picard says the vaccine protects against all strains. “That means the MMR (measles-mumps-rubella) shots children receive provide life-long immunity, with few exceptions.” Picard explains:
The confusion arises because the flu shot is required every year. That’s because flu viruses mutate and the strains that circulate can change markedly from year-to-year.
With measles, there are different strains, but very little variation between them, so the immune system recognizes them all as measles – if you have been properly vaccinated, or if you had the measles, and developed immunity.
People born before 1970 likely had measles and don’t need to be vaccinated. Those born between 1970 and 1979 probably had only one measles shot and likely need a booster. People born between 1979 and 1996 may or may not have had two MMR shots, depending on where they live. In some cases they may need a measles booster (or, more likely, a mumps booster). A two-dose MMR vaccine has been standard since 1996.
If you are unsure of your status, the easiest thing to do is get another shot. That is especially important if you are travelling to areas of the world with a lot of circulating measles, like India, the Philippines or France.
So why does it matter if there are new strains circulating?
Molecular genotyping of the type that identified the unusual strain in Toronto is done to facilitate global surveillance and outbreak investigations. If you know where the strain originated it’s easier to trace how people got infected and contain the spread.
For example, researchers know that an outbreak of 141 cases that has spread to 17 U.S. states (and Quebec) all originated at Disneyland, and that the strain is similar to strain that fuelled a large outbreak in the Philippines. The Ontario cases, on the other hand, have a different genetic fingerprint and the source has yet to be identified.
Since 1990, 19 different strains have been catalogued and sequenced: A, B2, B3, C1, C2, D2, D3, D4, D5, D6, D7, D8, D9, D10, D11, G2, G3, H1, H2. The Disneyland cases are a B3 strain.
A global measles sequence database, MeaNS, has been established at the Health Protection Agency in London, and contains sequence information from more than 10,000 measles samples.