What counts as “exposure” to measles?
Measles is transmitted primarily by tiny airborne respiratory droplets, which are emitted when people sneeze, cough, or exhale. A person can catch the measles from breathing air into which the virus has been released or by touching surfaces that have been recently touched by a person with the infection. In other words, transmission can happen between two people who never physically meet.
In 2013 guidelines for prevention and control of measles outbreaks, the Public Health Agency of Canada defined a “contact” as anyone who has: a) spent any length of time in a room or enclosed space with someone who had a confirmed case of measles during their infectious period; or, b) spent time in the same room as a person with a confirmed case of measles during their infectious period within two hours after that person left the room or space.
When is a person “infectious”?
The infectious period starts approximately four days before the onset of the rash and ends approximately four days after the onset of the rash. The rash typically begins about 14 days after exposure (so people become infectious about 10 days after exposure).
People who have compromised immunity (due to a congenital condition or cancer treatment, for example), can be infectious for the duration of the illness.
Who is considered “susceptible” to the measles?
According to the Ontario Ministry of Health and Long-Term Care Infectious Diseases Protocol, the following groups are not considered susceptible to measles:
People born before 1970; they likely had measles as a child.
People born in or after 1970 who are 18 years or older and have received at least one dose of the MMR (measles-mumps-rubella) vaccine. (This does not apply to people who work in health care settings, are military personnel, or are students in a post-secondary educational setting.)
People who work in a health care setting, are military personnel or are students in post-secondary education who have had two doses of the vaccine.
People between the ages of 12 months and 17 years who have received two doses of the MMR or MMRV vaccine, with the first dose given on or after the person’s first birthday and the second dose being given at least four weeks later (if MMR) or six weeks later (with MMRV).
People who are not in one of the above categories are considered susceptible to catching measles.
What if you’re not sure whether you’ve had the MMR vaccination?
Ideally, you have written documentation of your immunization history; in Ontario, it is recorded in a small yellow booklet. If you can’t find it and your doctor has no record, there is no harm in getting the vaccine again. “The wonderful thing about the MMR vaccine is you can have as many doses as you like,” says Natasha Crowcroft, chief of applied immunization research at Public Health Ontario. “The risk of getting an adverse effect actually goes down with the number of doses. So the more you doses you get, the less likely you are to have a side effect.” *(addendum below)
What is the role of public health agencies in an outbreak of measles?
Local public health units are alerted when a person is suspected of having measles, or is diagnosed with measles. The units then work with the patient (or the patient’s parents) to identify where the patient was and who they had contact with during their infectious period. Public health will then reach out to anyone who had contact with the patient in an effort to identify people who may be susceptible to infection.
For people considered susceptible, public health will make recommendations appropriate to the situation, for example: vaccination, a dose of immune globulin or isolation (more on these measures below).
Crowcroft points out that it can sometimes be hard to reach all the people the patient had contact with, or even to know who all those people are. A person is infectious before the rash appears, and other symptoms (such as a fever) may not keep them at home. That’s when notices go out alerting the public that if they were in a particular mall or airport on a particular day to contact public health. “We really rely on the media to get those messages out,” says Crowcroft.
Should someone who is susceptible and has been exposed to measles receive the MMR vaccination?
The vaccine can provide some protection if it is given within 72 hours of exposure, though “it’s not as good as being immunized beforehand,” Crowcroft says.
Babies between ages six and 12 months can receive the MMR vaccine within 72 hours of exposure, but they must still receive two more doses after age one in order to ensure long-lasting immunity to measles.
The live vaccine is not recommended for pregnant women, for babies under age six months, or for people who have compromised immunity. These groups can receive a dose of immune globulin (IG) within six days of exposure to help reduce the risk of infection.
Can IG help reduce the risk of infection for other susceptible people?
Yes. If a person has missed the 72-hour window for the prophylactic MMR vaccine, immune globulin may help mitigate the risk if received within six days of exposure.
Should a susceptible person who has been exposed stay home from school/work?
If the person does not want to, or has missed the window in which to receive a prophylactic vaccine or dose of IG, they can potentially transmit measles to others. Thus, it is recommended that they: stay away from anyone who is at high risk because they are very young, are pregnant, or have immune problems; that they stay home from daycare, school and work settings; and that they refrain from travel, beginning five days after first exposure and lasting up to 21 days after the last exposure.
Are the recommendations different when it comes to people who work in health care settings?
For people who work in health care who have been exposed to measles and have only had one dose of the MMR vaccine it is recommended that they receive a second dose and have their blood tested for immunity. While they are waiting for the results, it is recommended that they remain home from work from the fifth to the 21st day following exposure.
Does immunity diminish? Is there a way to check?
“It’s a really good question,” says Crowcroft, who researched the issue of waning measles immunity for the World Health Organization. “There is good evidence that your antibody levels wane, but what we haven’t found so far is evidence that they’re currently falling low enough to change how well the vaccine is working.” She explains that when a person is first vaccinated, they have high antibody levels, but these fall over time. “That doesn’t necessarily mean you’re not protected. They have to actually fall down to a pretty low level, and also it’s only one bit of your immune system, because you have cellular immunity as well as antibodies.”
It’s possible to have your immunity tested through blood work, but it may well show low antibodies, says Crowcroft. In which case, the recommendation would be another dose of vaccine. “So then you’ve had a blood test and you have a vaccine anyway,” she says. “Essentially it’s just a lot simpler—and it’s a smaller needle actually—to have another dose of vaccine than it is to be tested. So we don’t generally recommend testing.”
The current understanding, says Crowcroft, is if you’ve had two doses of the vaccine or you’re born before 1970, “you’re good to go.” But the vaccine is relatively new, and it’s still unknown whether it can protect people for a whole lifetime. “There’s no one who’s 80 who’s been vaccinated. Most people in their 50s didn’t get vaccinated, they got disease. We haven’t had communities of people who are in the second half of their lives who were vaccinated.” So there are some questions and concerns about waning immunity in the next couple of generations.
But that’s in the future, Crowcroft emphasizes. “Right now the biggest challenge, not just in Canada but really everywhere, is not that vaccines are failing, but that people are failing to get vaccinated.”
*Update, June 14, 2019: If someone is unsure about their immunization status, says Natasha Crowcroft, it is extremely safe to receive a second or third dose of the MMR vaccine. In fact, the frequency of adverse events such as fever and rash falls with increasing number of doses of live vaccines because once antibodies have been developed they prevent replication of the attenuated vaccine virus (whether measles, mumps, rubella or other live vaccine).
To illustrate, a review of literature conducted as part of a study in Vaccine by Abedi and colleagues found median rates of rash in different studies of 4.8 percent after first dose of MMR, 3.6 percent after second dose of MMR and 0.4 percent after third dose of MMR vaccination. Rates of fever were median 28.9 percent after MMR1, 8.7 percent after MMR2 and 1.3 percent after MMR3. These rates compare with the risk of getting measles, in which typically everyone gets a fever and rash with a measles infection.
In Ontario, a study in Paediatrics & Child Health found that the rates of all adverse events reported to public health in 2012–16 for 1–3-year-olds (the age group in which the first dose is routinely given) is 7.7 per 100,000 compared with 0.5 for 4–9-year-olds (when the second dose is given).
The World Health Organization and the Public Health Agency of Canada Canadian also note falling rates of adverse events with increasing number of doses on the WHO’s vaccine safety information sheet and on PHAC’s Immunization Guide.