However, as in other countries with high measles coverage rates, small-scale epidemics have been repeated in Korea since 2013. This was due to the import of measles from other countries. These events have also been reported in other countries where measles elimination has been confirmed.2

Even after the global high rate of measles vaccination has been maintained, the recurrence of small-scale epidemics has been attributed to factors such as vaccine cold chain handling issues, age-related vaccination policies, individual waning immunity or suboptimal immunity, and heavy exposure.3

Despite the high rate of measles vaccination, the common cause of measles epidemics is that measles neutralizing antibodies are reduced to less than protective immunity after more than 10 years after the second dose of measles vaccine. It is natural that they become vulnerable to measles if they are not given additional vaccination with low antibody. Therefore, there is a need for countermeasures against this problem.45

In a domestic study of this aspect, it was reported that immunity against measles was significantly lower in adolescents and young adults in their 20s.6 These age groups were children who were vaccinated at the time of the catch-up vaccination policy in 2001 as a result of confirmation of waning immunity after the passage of time. This study reaffirmed that measles waning immunity differs from that seen with long-term immunity due to cell-mediated immunity after wild infection.7 In countries where measles elimination is established solely based on measles vaccination, small measles epidemics will continue to occur in younger infants under the age of one who have not been vaccinated, and in aging populations with reduced defense immunity if a measles outbreak occurs in a foreign country.

What is the best control of measles outbreak in a country with high measles vaccine coverage?

In order to solve these problems, it is necessary to continuously carry out epidemiological studies on age-specific measles immunity in the countries concerned and to maintain the herd immunity by increasing the protection rate of population (PRP) by at least 93%. To achieve this, the claim that the immunity to measles should be increased through supplemented measles vaccination to the lower age group of PRPs should be reconsidered.8 Especially, infants under one year of age should be checked for the maintenance period of maternal measles antibody, and the minimum vaccination age should be estimated in case of epidemic and should be reflected in the appropriate vaccination schedule in the future. In addition, quarantine should be strengthened, and the measles surveillance and genetic analysis system should be firmly maintained to quickly confirm the genetic information of measles virus introduced from foreign countries.9

In countries where measles vaccination rates are high, such as in Korea, infected patients are not classic, but instead have modified measles, which makes it difficult to diagnose early.10 These patients can spread measles on a large scale. Therefore, strict isolation standards of measles should be applied at the time of the epidemic. Unvaccinated infants in the epidemic area should be vaccinated early and measles vaccination for unvaccinated young adults should be encouraged. In particular, in Korea, since the vaccination registration program has not been established in adults, their vaccination history cannot be known. Therefore, it is urgently necessary to develop an adult vaccination registration system in a short time.

Since the mid-1980s, vaccination policies have been very successful in Korea and most vaccine preventable diseases have been eliminated and well managed. However, in maintaining this condition, there is a tendency to show interest only when an outbreak occurs. In the current situation where measles elimination has been declared, it will be necessary to find and implement measures to continuously maintain the policy, and a method to maintain it, in consultation with experts.