MMR vaccines provide long-lasting protection against measles transmission

Sustained herd immunity against measles in Finland suggests that the measles-mumps-rubella (MMR) vaccine provides long-term protection against onward measles transmission, according to a new study conducted by researchers at the Finnish Institute for Health and Welfare (THL), the University of Turku and LUT University. Without long-term protection against transmission, Finland, despite its high vaccination coverage, would likely have experienced larger measles outbreaks, like those seen in several other European countries.

The researchers examined the relationship between measles antibody levels and herd immunity using data from a cohort of MMR-vaccinated individuals in Finland collected between 1982 and 2012. In Finland, the MMR vaccine is given in two doses, at around one year and six years of age.

Research often focuses on what level of antibodies is sufficient to prevent infection. However, for herd immunity it is essential that a sufficiently large proportion of the population does not transmit the virus onward. Even when vaccination does not fully prevent infection, it may reduce viral replication enough to limit onward transmission.

A “firewall population” prevents spread

The study introduces the concept of a “firewall population” — individuals whose immunity is sufficient either to prevent infection or to interrupt chains of transmission. This group acts as a firewall against viral spread and must be large enough for herd immunity to be maintained.

Based on the observation that Finland has not experienced large measles outbreaks since the 1980s, the researchers inferred what level of immunity is needed to belong to this firewall population.

They first estimated the rate at which antibody levels decline after vaccination. These estimates were applied across age groups to reconstruct population antibody levels for 2022. Based on these results, they estimated the size of firewall population under different candidate antibody thresholds. An antibody threshold was considered sufficient if it resulted in a firewall population large enough to maintain herd immunity.

Challenges in comparing antibody measurements over time

A key challenge in the study relates to the use of earlier results from a decades-long follow-up of MMR-vaccinated individuals, during which antibody levels were measured using multiple assays, including plaque reduction neutralization tests (PRNT), haemagglutination inhibition, and ELISA. Although calibration had been applied to harmonize results across assays, antibody levels varied systematically between assays. Even within a single assay, calibration may have differed between study rounds.

Despite applying conversion factors between assays to enable comparisons over time, uncertainty in absolute antibody concentrations remained. The estimated long-term trends in antibody levels in older age groups were also sensitive to how the rapid decline in antibodies in the years immediately following vaccination was accounted for in the analysis. As a result, the study identified qualitative relationships between antibody levels and the size of the firewall population but did not establish a single quantitative antibody threshold for preventing transmission.

Herd immunity sustained despite unclear antibody dynamics

Scenarios in which vaccinated individuals lost the ability to block transmission over time, causing the firewall population to fall below the herd immunity threshold, were inconsistent with the continued absence of measles outbreaks in Finland, indicating persistent protection against transmission. However, the study could not determine whether this protection persists even at relatively low antibody levels or whether antibody decline slows significantly over time.

Regardless of the underlying mechanism, the findings demonstrate that vaccination maintains population-level protection against transmission and that herd immunity can be sustained with sufficiently high coverage.

High vaccination coverage is critical

The results underscore the importance of maintaining high vaccination coverage to preserve herd immunity and prevent outbreaks. Comparability of antibody assays needs to be planned in long-term follow-up studies to assess protective immunity over time.