Sri Lanka has become free of the viral infection

On July 9, Sri Lanka became the fourth country in the Asian region — after Bhutan, The Maldives and Timor-Leste — to eliminate measles. Amidst an increase in the number of cases worldwide, especially in Europe, this came as an encouraging development. Measles is considered as eliminated when a country interrupts transmission of an indigenous virus for three years.

However, the viral infection has, since the beginning of 2018, seen a resurgence in 49 of the 53 countries in the World Health Organization (WHO)’s European Region. A total of 1,60,000 cases and more than 100 deaths were reported from these 49 countries between January 1, 2018 and May 30 this year.

The number of cases reported last year in the European Region, the highest in this decade, was thrice the number reported in 2017 and 15 times the number for 2016. With nearly 78,000 cases reported in the first five months of this year, indications are that the number will surpass last year’s.

Ironically, the sharp increase in the region came despite vaccination coverage for the second dose being at a record-high of 91% in 2018. So what could have caused the spike? According to the WHO, the vaccination coverage has “not been uniform across the region nor high enough to ensure herd immunity” to break the transmission cycle.

Pockets of low coverage

Further, though the coverage is high at the national level in many countries, there are pockets of low coverage in them, causing an “accumulation of susceptible individuals” that goes largely unnoticed till such time an outbreak occurs.

So how did Sri Lanka manage to accomplish this at this juncture even though, worldwide, there has been a 300% increase in the number of cases in the first three months of this year, compared to 2018? The answer lies in an increased, and a wider, vaccination coverage for both doses.

While global coverage for the first dose of vaccine has remained stagnant at 85% and for the second dose, it is still at 67%, the coverage for Sri Lanka has been above 95% for both the first and second doses. The vaccine is provided to children under the routine national immunisation programme.

Regular vaccination campaigns

The island nation, where measles is a notifiable infection, has also carried out periodical mass vaccination campaigns to reach the small pockets of unimmunised children. Sri Lanka also has strong surveillance in place.

However, the country did encounter a few bumps in its path. In 1984, it introduced measles vaccine in the national immunisation programme and set the goal of eliminating the infection. Though local outbreaks were reported, the annual incidence declined till the year 1999.

During Sri Lanka’s measles epidemic of 1999-2000, nearly 15,000 cases were reported following which a two-dose vaccine schedule was introduced, resulting in a sharp decline in incidence. The country reached an elimination target of less than five per one million people in 2011.

But in 2012, the measles immunisation schedule was changed following which babies no longer received measles vaccine at the age of nine months but a measles, mumps and rubella (MMR) vaccine on completing 12 months. Following this, in 2013, the country witnessed its last major measles epidemic. Besides other reasons, the change in immunisation schedule was seen as a possible cause.

The MMR vaccine schedule was again changed following a sero-survey data. Babies now began receiving the first dose at nine months and second dose at the age of three years. The country reported its last case of measles caused by an indigenous virus in May 2016.

prasad.ravindranath@thehindu.co.in