On October 3, 2019, the U.S. just about managed to retain its measles elimination status declared nearly 20 years ago. A month earlier, New York State declared the end of a measles outbreak, which began on October 1, 2018 and continued for almost a year, bringing the country very close to losing the status.
The last case of measles in New York State occurred on August 19 and completed 42 days (two incubation periods for measles) after the onset of rash. It ended just a couple of days before the duration of the outbreak could cross the one-year mark. This was crucial as a country loses the measles elimination status if a chain of transmission from a given outbreak is sustained for more than 12 months.
An outbreak in New York City, which began on September 30, 2018, led to more than 600 confirmed cases. The outbreak in nearby Rockland County, New York, started the next day and led to more than 300 cases. While 29 other States in the U.S. reported outbreaks in the past year, these did not last long. The reason why they were both limited in size and short-lived was mainly because the vaccination coverage was high leading to high immunity protection in the population.
Reasons for outbreak
The nearly year-long transmission in New York highlights the possibility of a sustained spread of measles in small pockets of an under-immunised community even when vaccine coverage with two doses nationally is high. Inequities in vaccine coverage, or gaps in vaccine coverage between communities, age groups and geographic areas in countries with high coverage at the national level, provide a fertile ground for outbreaks and for prolonged spread in such under-immunised groups.
Gaps and disparities in vaccine coverage between communities was the reason why the two outbreaks among the children of New York lasted for almost a year. Vaccine coverage among children belonging to the ultra-Orthodox Jewish community was not high; measles vaccination coverage in schools in the outbreak area was only 77%. In addition, there was also a delay in vaccination. The reason? Parents had refused to vaccinate their children fearing that the vaccine might cause autism. Low protection in children of this community meant that they ran a high risk of getting infected by unvaccinated people returning from countries with ongoing measles transmission.
While 1,249 cases of measles were laboratory-confirmed in 2019 from 22 outbreaks in 31 States, 75% of the cases were restricted to the Orthodox Jewish community in New York.
Problem in India
These details are important for India, which has a twin problem. The first is that it has huge pockets of under-immunised children. Second, the immunisation coverage with two doses at the national level is far below the World Health Organization level of 95% needed for protection and elimination. Intensified efforts to increase immunisation coverage in recent years have led to a sharp drop in the number of measles cases annually in India. Yet, in the October 2018-2019 period, India reported 71,834 cases, the third highest number in the world, according to the WHO.
While India intends to eliminate measles by 2020, the vaccination coverage has nowhere reached the 95% threshold for two doses. According to the June 2019 WHO and UNICEF estimate for national immunisation coverage, measles vaccine coverage in India in 2018 for the first dose was 90%. It was 80% for the second dose. But the reported coverage levels are “likely an overestimation”, the report cautions, based on a coverage evaluation survey.
Protection offered by maternal antibodies last for only four-five months, while the first dose of measles immunisation is at nine-12 months of age. Thus there is a huge window during which infants are vulnerable to measles infection. Also, about 15% of children in India fail to develop immunity from the first dose of measles vaccine. Till such time older children are fully protected with two doses, infants will remain vulnerable.