[T]he question arises whether the current level of herd immunity in Korea is now insufficient for protecting against measles infection.
Waning levels of measles antibodies with increasing time post-vaccination suggests that measles susceptibility is potentially increasing in Korea.
This trend may be related to limitations of vaccine-induced immunity in the absence of natural boosting by the wild virus, compared to naturally acquired immunity triggered by measles infection.
Owing to the occurrence of large, nationwide measles outbreaks with approximately 55,000 cases of measles and 7 deaths during 2000–2001, the government implemented the 5-year National Measles Elimination Plan…
2-dose MMR vaccination coverage had been maintained at >95% since 1996. In March 2014, the World Health Organization (WHO) verified that measles had been eliminated in Korea.
Although measles had been eliminated in Korea, the resurgence of measles outbreaks related to imported and import-associated measles cases occurred during 2013–2014. Most patients with measles were infants aged <1 year, but measles cases were also identified in patients aged 13–24 old who had received a 2-dose measles vaccination.
Measles outbreaks among highly vaccinated populations have been observed in many countries. Such outbreaks in a population with high 2-dose measles vaccine coverage may be related to a vaccine-handling issue (cold chain issue), the vaccination strategy (number of doses, age of vaccination), host immunity (waning immunity, suboptimal immunity), and environmental factor (heavy exposure).
By investigating the seroprevalence of measles in Korea, we provide a significant window into current measles herd immunity to better understand the prevalence of measles susceptibility underlying measles outbreaks in Korea.
During 2010–2016 in Korea, 36.2% of individuals with confirmed measles infection were unvaccinated, 46.8% were vaccinated previously (10.5% with 1-dose, 36.2% with 2-dose)…
The existence of potential factors underlying vaccine failure, such as waning immunity, was suggested by data generated in previous studies on measles outbreaks in highly vaccinated populations.
[D]eclining neutralizing measles-antibody titers were detected in both kindergarteners and middle schoolers, 5 years and 10 years after a secondary vaccine dose… in a study conducted in the United States.
[A]dolescents and young adults presented higher susceptibility rates compared to other age groups.
The group aged 30–50 years, whose immunity against measles was presumably acquired naturally by previous exposure to wild measles virus, presented higher seroprevalence and antibody concentrations than did other age groups except for the 1–12-year-old age group.
Our observations that long-term and higher antibody levels were present following natural infection than after vaccination agree with data from other previously published studies.
Several reports have warned that the susceptibility to measles infection may be rising because of waning vaccine-induced immunity over time after vaccination, in the absence of natural boosting by circulating measles viruses.
Our data showed good agreement between the incidence of measles and the susceptible age groups (adolescents and young adults) with measles seronegativity observed, suggesting the potential accumulation of measles-susceptible individuals in the population due to waning immunity, which may pose increased risk for measles outbreaks…
Cell-mediated immunity [not provided by vaccines] may protect against measles virus infection by promoting viral clearance, recovery from acute disease, and the persistence of long-term immunity.
[P]assively acquired maternal measles antibodies declined significantly and expired at 8 months after birth in Korean infants.
Several studies of early waning of maternal measles antibodies in infants were published in recent years, and such waning may be related to low maternal measles titers, a limitation of vaccine-induced immunity compared with naturally developed immunity after wild measles virus infection, and the absence of natural boosters.
Because an increasing number of women have acquired immunity by vaccination instead of natural measles infection due to decreasing opportunities for wild virus exposure, the immunity gap in measles protection occurring between the loss of passive immunity derived from the mother and immunity acquired from the first vaccination can be amplified.
As a consequence, the proportion of infants susceptible to measles infection increases progressively.
The waning of measles antibodies in adolescents and young adults after vaccination and in infants after birth but before the first vaccination, and the limitation of vaccine-induced immunity in measles-eliminated environments versus acquired immunity by natural infection were confirmed in our age-specific seroprevalence study. These findings suggest that an increasing proportion of measles-susceptible individuals is occurring with increasing time post-vaccination in Korea.