Background: The authors have added the second dose of measles vaccine to children aged 18 months since 1997 because of the measles outbreaks in Nan province in 1993-1994.

Objective: To compare measles antibody level between two doses vaccination at 9, 18 months and single dose at 9 months in children at the age of 4 to 6 years old.

Material and Method: A cross sectional serological study in children 4 to 6 years old was performed between August 2008 and August 2009 at three hospitals in Nan and Phrae provinces. The subjects were divided into two groups, 1) 100 children in Nan provincial hospital received two doses of measles vaccination at the age of 9 and 18 months and 2) 91 children received single dose measles vaccination at the age of 9 months, 41 from Phrae provincial hospital and 50 from Weingsa district hospital. Blood samples were drawn for measles antibody measurement by ELISA assays at Virus Research Institute, National Institute of Health, Thailand.

Results: The mean measles antibody level in children 4 to 6 years old in both groups was a satisfactory high level, 1,887.67 and 1,621.02 mIU/ml in single and two doses vaccination respectively, which were not statistically significant (p = 0.431). The higher level in single dose group could be explained by the average age being younger than the two doses group by one year (4 years 2 months vs. 5 years 4 months). Therefore, the waning immunity in younger age group is suspected to be less than the older age group. The rate of protective measles antibody level (> 255 mIU/ml) was significantly higher in the two doses group than the single dose, 87% compared to 76% (p = 0.046), which represented primary vaccine failure at the age 4 to 6 years of 13% and 24%, respectively.

Conclusion: The authors suggest that a second dose of measles vaccine at the age of 18 months be administered to decrease the number of primary vaccine failure from 24% to 13%. Further studies in the same age group and in different areas are required to confirm these findings.

Keywords: Comparison study, Measles antibody, Single or two-dose measles vaccination