Seropositivity of Measles Antibodies among the Israeli Population -

רוית בסל ¹ Viki Indenbaum ² Rakefet Pando ^1,2 Tal Levin ² Eilat Shinar ³ Doron Amichay ⁴ Mira Barak ⁵ Anat Ben-Dor ⁶ Adina Bar Haim ⁷ Ella Mendelson ^2,8 Dani Cohen ⁸ Tamy Shohat ^1,8

¹Ministry of Health, Israel Center for Disease Control, Israel
²Public Health Services, Central Virology Laboratory, Israel
³Magen David Adom, Blood Services, Israel
⁴Clalit Health Services, Soroka University Medical Center, Israel
⁵Clalit Health Services, Haifa and Western Galilee Laboratories, Israel
⁶Clalit Health Services, Schneider Children Medical Center, Israel
⁷Mayanei Hayeshua Medical Center, Mayanei Hayeshua Medical Center, Israel
⁸Sackler Faculty of Medicine, Tel Aviv University, Department of Epidemiology and Preventive Medicine, Israel

Introduction: Measles vaccine is administered as part of the routine childhood immunization program at one year and 6 years of age. Although measles outbreaks are less common since the introduction of the vaccine, we experienced a large scale measles outbreak since the beginning of 2018. It is therefore of great importance to know the immunity status of the Israeli population against measles.

Methods: 3,164 serum samples from the Israel Center for Disease Control National Serum Bank collected throughout 2015 were tested for measles antibodies. All tests were performed at the Central Virology Laboratory using ELISA commercial kit (Enzygnost, Anti-Measles Virus/IgG: Behring, Marburg, Germany), according to the manufacturer`s instructions. Intermediate test results were classified positive after demonstrating protective titers in a neutralization test.

Results: The overall seropositivity rate for measles was 90.7%. Seropositivity rate at younger than 6 months was 48.9%, representing maternal antibodies and decreased to 3.8% among infants aged 6 months to 11 months. Following the administration of the first dose of the vaccine, seropositivity increased to 90.7% in the age group 1-4 years, and reached 96.1% in the age group 5-9 years, mostly vaccinated with two doses. In the age groups 10-44 years, seropositivity was high (around 90%) and reached 93.8% in the age group 45-59 years. Evidence for natural immunity was observed in the age group 60 and older, where 98.9% of all sera samples tested positive. A relatively high amount of intermediate results (~15%) was found in population 15-44 years old.

Conclusions: Our results suggest high immunity in the Israeli population against measles. Infants younger than 1 year are at high risk for contracting measles especially between 6 months and 11 months. The observed outbreaks in Israel may be attributed to specific population sub-groups with low immunization coverage, which our sero-survey was not able to identify.