Measles in adults and children in the last few months in California have brought home the fact that the extremely contagious virus is in our backyard.
Once under control and even eradicated, measles is making a strong comeback, including almost 50 cases in the state, the highest since 2019, and almost 1,800 nationally as of April, marking the largest surge in decades.
Charles Chiu , MD, PhD, UC San Francisco professor of Laboratory Medicine and Medicine, in the Division of Infectious Diseases, explains how measles spreads, why it is so difficult to contain, and who is most at risk for serious illness. He covers how effective the MMR vaccine is and why maintaining high vaccination rates is essential to preventing further outbreaks.
What makes measles so contagious?
Measles is, quite simply, one of the most contagious infections we know. In an unvaccinated population, a single infected individual can transmit the virus to 12 to 18 others, which is even higher than the most transmissible COVID-19 variants. What makes it especially challenging is that people can spread the virus before they develop symptoms, allowing it to move through a population very quickly.
How does measles spread?
Measles spreads through the air via tiny particles released when an infected person coughs or sneezes, which can be inhaled by others. These particles can remain in the air for up to two hours, which means someone can become infected simply by entering a space where an infected person has recently been, even after that person has left.
What are the symptoms?
The incubation period for measles is typically seven to 14 days. Measles is not easily detected, as early symptoms of fever, cough, runny nose, and red eyes can resemble a common respiratory illness. The classic measles rash usually appears a few days later.
Why is measles so difficult to control?
A person can be asymptomatic before symptoms appear; the virus spreads through the air and lingers; and it is highly contagious. Without strong vaccination coverage at the population level, cases can grow exponentially.
How effective is the MMR vaccine?
The MMR (measles, mumps, and rubella) vaccine is highly effective in preventing the spread of all three diseases. After one dose, protection is about 93%. After two doses, it rises to about 97%. Importantly, even in the rare case that a vaccinated person becomes infected, the illness is typically much milder and much less likely to lead to serious complications.
What is a titer?
A titer is a blood test that measures antibody levels against a specific pathogen such as measles to determine whether a person has immunity. It can be used to assess an individual's immune response but does not measure overall vaccine effectiveness.
What is herd immunity, and why is a high level required to keep measles under control?
Herd immunity means that enough people in a population are immune to prevent the virus from spreading. Because measles is so infectious, about 95% of the population needs to be immune to stop transmission. With a virus that spreads this easily, even a small number of unvaccinated individuals can lead to an outbreak.
Why are measles cases increasing again?
What we're seeing now is largely driven by declining vaccination rates in certain communities, which allows the virus to spread rapidly like a wildfire through the population. Increased global travel also contributes to the potential spread of the virus nationwide and globally.
Why do outbreaks tend to occur in clusters?
Epidemiologic data shows that the virus spreads most efficiently in areas where vaccination rates are lower.
Who is most at risk for severe illness?
The highest-risk groups include young children, particularly those under age 5, as well as those who are immunocompromised, such as transplant recipients. Older adults are also at increased risk. These groups are more likely to develop severe complications if infected.
How serious can measles be?
Measles can be life-threatening, with complications that include pneumonia, brain inflammation (encephalitis), and death. Although relatively rare, roughly 1 to 3 people per 1,000 cases die from the infection, with higher risk among those who are unvaccinated, very young, or immunocompromised.
Does the measles virus mutate, and does that affect the efficacy of the vaccine?
Yes, measles is an RNA virus and it does mutate. However, its mutation rate is much lower than that of viruses like influenza or COVID. High vaccination coverage has also limited circulation of the virus, reducing opportunities for it to accumulate significant changes. As a result, it has not evolved in a way that meaningfully reduces vaccine effectiveness, which is why the same vaccine has remained effective for decades.
What treatments are available for measles?
There is no specific antiviral treatment for measles. In severe cases, patients may require hospitalization, including intensive care support. Most cases can be managed by staying hydrated and keeping any fever under control. The World Health Organization and U.S. Centers for Disease Control and Prevention (CDC) also recommend vitamin A supplementation in children, as it may reduce complications and mortality.
Do masks help prevent measles?
Masking is not a fail-safe protection against measles, although it can reduce the risk of infection. The most effective protection is provided by a well-fitting respirator mask, such as an N95 mask. Powered air-purifying respirators can provide an even higher level of protection, particularly in health care settings. Standard masks are much less effective given how easily measles spreads.
What needs to happen to bring measles back under control?
The most important step is increasing vaccination uptake. The MMR vaccine is highly effective, and widespread vaccination is the most powerful tool we have to prevent outbreaks. Without high vaccination rates, measles will continue to circulate and could become endemic again.
