Measles is highly contagious, but vaccine-preventable
Canada is seeing a resurgence of measles, with cases in the first quarter of 2024 already far surpassing the total for all of 2023. There were 12 cases last year, and more than three times that number so far in 2024, with 38 reported as of March 19.
Most of these cases (28) are in Québec, and eight are in Ontario, while Saskatchewan and British Columbia have each reported one case.
As an immunologist with a focus on host-microbe interactions and antiviral immunity, I have been following recent measles outbreaks.
SYMPTOMS AND COMPLICATIONS
Measles (also known as rubeola) is a serious but vaccinepreventable disease caused by an RNA virus of the family Paramyxoviridae.
Measles usually begins with fever, runny nose, conjunctivitis (red watery eyes), sore throat and coughing. These can be initially mistaken for common cold or flu.
However, these non-specific signs and symptoms are typically followed by clinical manifestations that are characteristic of measles. These include Koplik spots (tiny white specks with bluishwhite
U.S. CENTERS FOR DISEASE CONTROL centres in the inner lining of the cheeks), and subsequently a measles rash appearing on the face before spreading downward to other areas of the body.
In most cases, measles resolves on its own. However, severe complications may arise, especially in immunocompromised individuals. Complications can include pneumonia, encephalitis (brain inflammation and swelling), blindness, deafness and permanent neurological consequences. When measles occurs during pregnancy, it can result in miscarriage, premature labour, stillbirth, birth defects or even fetal death. The most severe cases of measles can be fatal.
Also of note, infection with the measles virus can weaken the immune system for months or years, increasing the risk of infections with a wide range of microbes. While measles-associated immunosuppression has been documented for decades, we are only beginning to decipher its underlying mechanisms. For example, a phenomenon called “immune amnesia” is thought to contribute, at least partially, to unrelated infections in the aftermath of measles.
WHAT IS IMMUNE AMNESIA?
The naturally occurring (wild-type) strains of measles virus can target, infect and kill memory B and T lymphocytes, which are instrumental to antimicrobial defence. This is because one of the three measles virus receptors, called CD150, happens to be abundantly present on the surface of these lymphocytes.
Long-lived memory cells, which accumulate as a result of immunizations and infections over time, remain in a poised state to mount rapid and rigorous recall responses when we re-encounter microbes. B cells orchestrate the production of antibodies that neutralize extracellular microbes, and T cells work to destroy infected cells. Therefore, when people lose their precious memory cells to measles, the immune system is set back to a default mode, as if it has never seen any microbes or vaccines in the past.
To add insult to injury, measles virus may also eliminate “memory-like” innate T cells, which also express CD150, thus removing yet another potent weapon from our antimicrobial arsenal. Therefore, collectively, the ability of measles virus to find and kill memory and memory-like lymphocytes can lead to adaptive and innate immune amnesia, rendering a measles patient or survivor prone to many opportunistic infections.
HOW DOES MEASLES SPREAD AND HOW CONTAGIOUS IS IT?
Measles virus spreads easily through airborne droplets released by infected people when they breathe, talk, laugh, cough or sneeze. In addition, measles virus infectious particles can remain active in the air and on contaminated surfaces for up to two hours.
Measles virus is one of the most contagious respiratory pathogens known, with each person with measles passing on their infection to 12 to 18 other people in a susceptible population. Measles virus is more transmissible than influenza viruses and SARSCOV-2
variants.
HOW EFFECTIVE ARE MEASLES VACCINES?
Measles vaccines are safe, affordable and extremely effective. According to the World Health Organization (WHO), measles claimed 2.6 million lives each year before 1963 when a measles vaccine became available. Since then, widespread immunization programs have saved millions of lives, including an estimated 56 million just between 2000 and 2021.
Measles vaccines contain a live measles virus strain that has been attenuated so that it does not inflict harm; yet, it is sufficient to generate protective immune responses.
The measles-mumpsrubella (MMR) vaccine or the measles-mumps-rubellavaricella (MMRV) vaccine is routinely administered to children in two doses, with a first dose being given after the first birthday, typically between 12-15 months of age, followed by a booster dose recommended after 18 months of age and before attending school. This should afford lifelong protection against measles in most people.