Long-term immunity induced by measles vaccine

The resurgence of measles in the USA has reignited interest in long-term vaccine-induced immunity. Are those vaccinated in the 1960s and 1970s still protected today?

Measles is still a current challenge

In early 2024, the United States experienced a sharp increase in measles cases, with clusters reported in multiple states, including Florida, Pennsylvania, and Illinois. This resurgence, while still limited in absolute numbers, has exposed significant gaps in herd immunity and reawakened concerns about waning vaccine protection in adults. Although the majority of cases have occurred in unvaccinated children, some infections were documented in adults who believed themselves immune. This has led many clinicians and public health experts to revisit a fundamental question: do adults vaccinated decades ago still have protective immunity?

Measles is a highly contagious viral disease caused by a paramyxovirus of the genus Morbillivirus. It typically presents with a prodrome of fever, cough, coryza, and conjunctivitis, followed by the characteristic maculopapular rash that begins on the face and spreads caudally. Complications are not uncommon, especially in young children and immunocompromised individuals, and can include otitis media, pneumonia, and encephalitis. Before the advent of vaccination, measles caused millions of deaths globally each year. The introduction of effective vaccines transformed its epidemiology, making elimination a realistic goal in several regions.

Measles vaccines are effective and safe

The first measles vaccines became available in the early 1960s. Initially, both inactivated and live attenuated formulations were used. The inactivated vaccine, introduced in 1963, was later found to induce only short-term immunity and was associated with an atypical measles syndrome in some recipients. It was withdrawn from the market in 1967. The live attenuated Edmonston B strain, also introduced in the early 1960s, was effective but had high reactogenicity. A further improved version, the Edmonston-Enders (Moraten) strain, became widely available in 1968 and forms the basis of current formulations. Since the late 1980s, a two-dose schedule has become standard to close the gap of primary vaccine failure.

Today, measles vaccination is typically administered as part of the MMR (measles, mumps, rubella) or MMRV (with varicella) vaccine, using the Moraten strain. In the United States, the first dose is recommended at 12-15 months of age and the second at 4-6 years. In Europe, similar schedules are adopted, though enforcement varies. Italy introduced mandatory measles vaccination for children in 2017, while Germany enacted a federal law in 2020 requiring proof of measles immunity (through vaccination or serology) for all schoolchildren and childcare workers born after 1970. These regulations reflect growing concern about declining vaccination coverage and the risk of endemic transmission.

Measles vaccines provide lasting immunity

Numerous studies have evaluated the duration of immunity conferred by the measles vaccine. Humoral immunity, as measured by IgG antibodies, has been shown to persist for decades in the vast majority of vaccinated individuals. A landmark study by Amanna et al. (2007) found that antibody levels against measles remained stable over a period of 26 years, with an estimated half-life of over 3.000 years. Similarly, a UK follow-up study tracking individuals vaccinated in 1964 demonstrated 92% vaccine efficacy even after 27 years. These findings suggest that immunological memory, both humoral and cellular, is long-lasting and generally protective.

Nevertheless, waning immunity has been documented in a minority of individuals. Serosurveys in healthcare workers and university students indicate that up to 15-20% of those vaccinated decades earlier may lack detectable IgG titers, especially if they received only a single dose. Importantly, even in these cases, the immune system often retains memory: a single booster dose can elicit a rapid and robust anamnestic response in over 90% of seronegative adults. This has led some experts to propose targeted serological screening in high-risk groups rather than universal revaccination.

Do adults need a measles booster?

So, do people who were vaccinated against measles between 1960 and 1970 need a booster today? The answer depends largely on the type of vaccine they received. Individuals vaccinated between 1963 and 1967 may have received the inactivated formulation, which is now known to be insufficient for long-term protection.

The CDC recommends that such individuals receive at least one dose of the current live attenuated MMR vaccine. Those vaccinated after 1968 likely received the Moraten strain and, if they had two documented doses, do not require any additional booster. For individuals with uncertain records or who only received one dose, a second dose is safe and recommended in many situations, particularly for travelers, healthcare workers, and contacts of immunocompromised patients.

It is worth noting that individuals born before 1957 are generally presumed to be immune to measles, having likely acquired natural infection in childhood, when the virus was endemic. This assumption is supported by epidemiological data and has been consistently adopted in vaccination guidelines across the United States and Europe. As a result, routine serological testing or revaccination is not recommended for this cohort, unless they belong to high-risk categories such as healthcare workers or immunosuppressed individuals.

In summary, the available evidence indicates that measles vaccination provides durable immunity in most cases, especially when two doses of the live attenuated vaccine have been administered. However, the earliest vaccine recipients—those vaccinated before 1968—may not be adequately protected and should be revaccinated. In light of recent outbreaks, it is reasonable for clinicians to assess measles immunity in at-risk adults, especially those with incomplete records or in high-exposure settings. Serological testing can guide these decisions, but in the absence of reliable documentation, administering a booster dose remains a safe and effective strategy.

As measles remains a persistent threat in both high- and low-income countries, ensuring immunity in all age groups is crucial for elimination goals.

Sources
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