April 20 – While measles is largely preventable through vaccination, a small number of vaccinated individuals may still contract the illness, particularly during widespread outbreaks. These occurrences, referred to as “breakthrough infections,” are rare but raise important questions about vaccine effectiveness, immune system variability, and public health strategy.
A Look at the Ongoing Measles Outbreak
Since the start of 2025, more than 800 measles cases have been reported across the United States. A significant portion of these infections has been concentrated in West Texas, where an ongoing outbreak has persisted since January. Most of the individuals who contracted the illness had not received the vaccine. However, a small percentage approximately 3% were individuals who had received one or both doses of the measles, mumps, and rubella (MMR) vaccine.
Understanding the MMR Vaccine’s Effectiveness
The MMR vaccine remains one of the most successful tools in preventing measles. A single dose provides approximately 93% immunity, and receiving a second dose increases the protection level to nearly 97%. Still, even with such high efficacy, a small margin of individuals may not develop full immunity. This means that out of every 100 people fully vaccinated, approximately three could still be susceptible if exposed to the virus.
Health experts stress that this does not indicate a failure of the vaccine. Instead, it highlights a natural variation in how individual immune systems respond to vaccines. While most people build a strong and lasting defense, a few may generate a weaker immune memory despite full vaccination.
Why Do Breakthrough Cases Occur?
There are several reasons why a fully vaccinated person might still get measles. One key factor is individual immune response. Some people, due to genetic differences, do not produce the same level of antibodies even after receiving both doses of the vaccine. These people may respond to fewer parts of the virus, known as “epitopes,” limiting their immune system’s ability to fight off future infections.
Another explanation could involve the timing of vaccination. It usually takes the immune system about two weeks after getting vaccinated to develop strong enough defense against the virus. If a person is exposed to the virus shortly before or after receiving the vaccine, they may not have developed full immunity yet. This creates a short period during which a person may still be at risk of infection.
In rare situations, individuals may receive only one dose, thinking they are fully protected. A single dose still provides substantial defense but does not offer the same level of assurance as the full two-dose schedule. Breakthrough cases in this group tend to have more pronounced symptoms than those with two doses.
Characteristics of Breakthrough Infections
Even when measles occurs in vaccinated individuals, the illness is often milder than in unvaccinated cases. These modified measles cases may include a lighter or less widespread rash, and the classic high fever often above 104°F—may be absent or lower in intensity. Other typical symptoms like cough, runny nose, and red eyes may still occur, but they’re often less severe.
One of the key concerns with breakthrough infections is that, although they are generally milder, they can still be contagious. Individuals are considered infectious from four days before until four days after the rash appears. This means even vaccinated people can inadvertently spread the virus if infected.
Duration of Immunity: Does It Wane Over Time?
There has been ongoing debate about how long immunity from the MMR vaccine lasts. Scientific evidence strongly suggests that in most people, the protection provided by two doses is long-lasting, often for life. Measles is a stable virus that hasn’t mutated significantly over time, which is why the MMR vaccine continues to be effective after decades of use.
However, some research indicates that in rare cases, the immune system may gradually stop producing enough protective antibodies over time. This decline in immunity could occur naturally in certain individuals, leading to susceptibility decades after vaccination. While this is not common, it may explain a portion of the breakthrough cases in older adults.
Special Populations at Higher Risk
Certain individuals are naturally more vulnerable to breakthrough infections. Those with weakened immune systems, such as patients undergoing chemotherapy or people with autoimmune disorders, may not develop a strong immune response to the MMR vaccine. Similarly, individuals born with underlying immune deficiencies might be unable to mount an adequate defense even after vaccination.
Additionally, individuals who received the earlier versions of the measles vaccine between 1957 and 1968 may have incomplete protection. These vaccines were less effective compared to the current live-attenuated version used today. Health professionals often recommend that individuals born during that time consider a booster shot, especially if they are traveling or live in areas experiencing outbreaks.
When Should You Consider a Booster?
For individuals unsure of their vaccination history or concerned about waning immunity, it may be worth consulting with a healthcare provider. Those who received only one dose or were vaccinated during earlier decades could benefit from an additional MMR dose, particularly if they are in high-risk areas or frequently travel internationally.
It is important to note that receiving an additional MMR dose is safe, even for those who are already immune. There are no adverse effects from receiving an extra dose, making it a safe precautionary measure for those uncertain about their immunity status.
Long-Term Outlook on Measles Prevention
While breakthrough infections are possible, they remain relatively rare and are not a sign that the MMR vaccine is ineffective. The current outbreak continues to be driven largely by unvaccinated individuals. Nonetheless, breakthrough cases highlight the importance of maintaining high vaccination coverage, monitoring immune response, and staying informed about changes in public health guidance.
Vaccination remains the most powerful defense against measles, protecting individuals and communities alike from what can be a highly contagious and dangerous disease.