This study represents a nationwide seroprevalence study of measles IgG antibodies in young Thai men. The overall, by province, and by age cohort measles seroprevalence in these young men was below the 93–95 % recommended by the WHO for population immunity . These findings may explain partially the measles outbreaks occurring in adult populations in Thailand. Our study is consistent with previous measles seroprevalence studies conducted in Thailand. A study of a Thai population sample from the same birth years (1988–1992) had a similar measles seroprevalence .
We found significant difference of seroprevalence and GMT amongst the four regions of Thailand where the South exhibited the highest and the North the lowest. Although the geography of the Northern region of Thailand is mountainous with population scattered amongst hilly areas  whereas the Southern region is a peninsula between seas with population concentrated in the coastal areas, with limited regional vaccine coverage data in the early years of measles immunization as well as inconsistent case reporting by region; we cannot determine why measles seroprevalence is highest in the Southern region [5, 6, 13].
This study population consists of young Thai men born as the measles vaccination program was initiated in Thailand. The measles vaccine coverage ranged from 0 % in the oldest age group born before measles vaccine program initiation to a reported coverage of 73.0 % in the youngest age group in Thailand (Fig. 2). The higher seroprevalence and GMT were found in the oldest age group was the result of natural infection (born before vaccination), while the successively lower seroprevalence and GMT in the younger age groups was from a combination of vaccine induced immunity and natural infection. However, these observations are limited by lack of individual measles vaccination and infection history in this population.
Low seroprevalence of measles antibodies may be from inadequate vaccine coverage, vaccination failure, or waning immunity. Inadequate vaccine coverage may be caused by several factors such as lack of vaccine access, missed vaccination opportunities, and ignorance of the importance of vaccination. Vaccination failures may be attributable to the immaturity of the immune system, residual maternal antibodies, inadequate vaccine dose, or vaccine inefficacy . The seroprotective level increases from 85 % in children given first dose vaccine at 9 months to 95 % at 12 months. According to the EPI in Thailand, children were vaccinated with the first dose of the measles vaccine at the age of 9 months as recommended by WHO for countries with ongoing measles transmission; previous serological studies in Thailand showed that the passive maternal antibody was rapidly lost in infants starting from 4–6 months and almost completely absent at 8–9 months . A second later dose may cover the primary non-responders and unvaccinated persons who would otherwise accumulate over time and allow transmission . This may explain why Thais are still susceptible to measles infection with 40 % of reported cases found in children of pre-school age (2009–2013). Recently, the Ministry of Public Health, Thailand announced that from 2015 onwards the 2nd dose of vaccination previously given to first graders (6 years of age) will be given to children at 2.5 years of age instead . The waning of protective immunity could pose a risk in communities receiving a single dose of immunization at a young age without ongoing natural exposure . A lower measles seroprevalence than vaccination coverage found in this study may reflect a combination of waning immunity and measles vaccine failure in this Thai population. Usually waning immunity is age related in populations without natural infection or supplemental vaccinations, but this is not seen in our study cohort .
In Thailand, immunizations are routinely administered to children regardless of gender and no supplementary measles immunizations were given to the recruits upon entering the RTA; this nationwide measles seroprevalence study in a sample of Thai recruits selected by lottery at the district level may represent the measles protective immunity levels in the general young adult population of the same age groups . Our results show that approximately 20 % of Thai recruits in 2007–2008 are susceptible to measles; we speculate that females born in the same years may present similar trends in measles seroprevalence raising concerns for young Thai women of reproductive age even through a slightly higher measles seroprevalence in females has been reported in studies elsewhere [19–21].
With low herd immunity, measles infection can spread easily in densely populated communities such as prisons, dormitories, recruit camps and schools. WHO’s recommendation for immunization coverage sufficient to reduce measles mortality is ≥90 % at national level and ≥80 % at every district level. For elimination, each district must achieve vaccine coverage ≥95 % through a two-dose vaccination program . In 2001, a measles outbreak in Bangkok amongst healthcare workers emphasized the ease of transmission within high-risk adult populations . A supplementary immunization activity for higher risk adult communities may be an effective preventive measure, as implemented in Norway. Even with the two doses of MMR vaccine at the ages of 15 months and 12–13 years as part of Norway’s national childhood immunization program where vaccine coverage was higher than 95 %, sporadic incidence of measles was still reported [22, 23]. An additional dose of mump-measles-rubella vaccine has also been incorporated into the required vaccination program for military conscripts in Norway as an outbreak prevention measure .
Limitations of this study include a single sample collection from 2007–2008 in a population of all male Thai recruits. The lack of information on individual childhood measles as well as measles vaccination history and the measles vaccine strains limits the conclusions of this study. For future studies, it will be important to compare seroprevalence data on samples collected more recently from individuals with documented immunization histories from several age groups in the general population to better represent current Thai population protective immunity.