Though measles vaccine coverage is high in the U.S., the reason why the two outbreaks in New York City and New York State lasted for almost a year boils down to gaps and disparities in vaccine coverage between communities.
Nearly 20 years after being declared measles-free, on October 3, 2019, the United States escaped by a whisker to retain the measles elimination status when New York State declared the end of the measles outbreak. Exactly a month prior to that, New York City had declared the end of its outbreak. The measles outbreaks that started last year in New York City (September 30, 2018) and New York State (October 1, 2018) continued for almost a year bringing the U.S. tantalisingly close to losing the status.
The last case of measles in New York State outbreak occurred on August 19 and had completed 42 days (two incubation periods for measles) after the onset of rash just a couple of days before the duration of the outbreak could cross the one-year mark.
A country loses the measles elimination status if a chain of transmission from a given outbreak is sustained for more than 12 months.
The outbreak in New York City led to more than 600 confirmed cases while in New York State it caused more than 300 cases. While 29 other States in the U.S. had reported outbreaks in the past year, these outbreaks did not last long. The reason why measles outbreaks in other States were both limited in size and short-lived was mainly because the vaccination coverage was high leading to high immunity protection in the population.
What led to spread of measles in New York
The nearly year-long transmission in New York highlights the possibility of sustained spread of measles in small pockets of under-immunised community even when vaccine coverage with two doses nationally is high. Inequities in vaccine coverage, gaps and disparities 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 in New York children 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 children due to fear and misconceptions that the vaccine might cause autism.
The 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.
Measles cases, vaccine coverage in India
India has a twin-problem — huge pockets of under immunised children and the immunisation coverage with two doses at the national level being far below the WHO 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, during October 2018-2019 period, India reported 71,834 cases, the third highest number in the world, the World Health Organisation says.
While India intends to eliminate measles by 2020, the vaccination coverage has nowhere reached the 95% threshold for two doses. According to June 2019 WHO and UNICEF estimate for national immunisation coverage, measles vaccine coverage in India in 2018 for the first dose was 90% and 80% for the second dose. But the reported coverage levels are “likely an overestimation” it cautions based on a coverage evaluation survey.
Protection offered by maternal antibody 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.