The outbreak of measles in Kerala calls for a reassessment of the epidemiology of the disease

As Kerala grapples with measles outbreaks across districts, particularly Kozhikode, Malappuram and Palakkad, public health experts are calling for a detailed review of the epidemiology of the disease, which appears to have changed significantly over the years.

Once a predominantly childhood disease affecting children between the ages of one and five, measles is now reported across age groups, including fully vaccinated individuals and infants younger than nine months who are not yet eligible for routine measles vaccination.

The last major increase in cases was in 2022-23. As of Nov. 6, the state’s Integrated Disease Surveillance Program reported 824 confirmed and 1,681 probable measles cases. Five measles-related deaths have been reported, although only one has been confirmed so far.

In UIP

Measles vaccination was introduced into the Universal Immunization Program (UIP) in 1985 with a single dose given at nine months. A second dose, at 18 months, was introduced in 2010 to address the gap in immunity after the first dose and promote the elimination of measles.

The nine-month threshold was to ensure that maternal antibodies transferred across the placenta did not interfere with the vaccine-induced immune response. However, the changing age profile of recent outbreaks raises questions about whether this protection strategy remains adequate.

“In recent outbreaks, we have seen a large number of infants who developed measles before nine months, potentially because mothers of the current generation have lower levels of measles antibodies. We also have a large number of cases among older adults,” says KK Purushothaman, retired professor of pediatrics at Government Medical College, Thrissur.

He believes the state should consider an additional dose of the measles-containing vaccine under the UIP, either during adolescence or for women before marriage. The state’s technical advisory group on immunization should examine the proposal and commission serology studies among adolescents and young women to assess their levels of protective antibodies, he says.

The concern is supported by findings elsewhere. After the large measles outbreaks in Mumbai in 2023, in which more than 31% of reported cases occurred in infants under nine months of age, a seroprevalence study found that only 66.8% of mothers had sufficient measles antibodies to protect their infants.

Measles antibodies

An informal technical consultation report from the World Health Organization (WHO), published in the journal Vaccine in May 2025, acknowledged that women who acquire immunity through vaccination rather than natural infection may pass on lower levels of measles antibodies to their children. However, the experts concluded that the evidence was insufficient to change the current recommendation for the first routine dose at 9-15 months.

Meanwhile, research from closer to home suggests that vaccination history alone may not fully explain the emerging picture. The Rajiv Gandhi Center for Biotechnology (RGCB) study, which included 684 children aged four to 18 from Kerala and Tamil Nadu who had received at least two doses of measles-containing vaccine, assessed their measles antibody levels. Antibodies were also measured in 544 mothers.

A study published in JAMA Network Open in August 2025 found that 8.5% of children lacked antibody levels classified as protective. Another 89 children (13%) tested positive for measles-specific IgM, which may indicate recent infection, while 90 (13.2%) had borderline results. The researchers interpreted these findings as an indication of recent exposure to measles and a possible breakthrough infection.

Notably, 435 children (63.6%) received the third dose of vaccine, while seven received the fourth, but the antibody responses varied. The findings raise questions about the durability and adequacy of vaccine-induced immunity, although antibody levels alone do not determine whether an individual will develop clinical measles.

Keralam generally reports around 95% coverage with the first dose of measles-rubella (MR) vaccine. Coverage with the second dose, given at 18 months, was consistently lower, around 85-88%, with some districts reporting even worse coverage.

Incomplete immunization

Incomplete immunization remains the main cause of outbreaks. The health department has often attributed outbreaks to districts like Malappuram and Kozhikode, where there are significant gaps in vaccination. However, experts point out that there are areas of insufficient immunization across the state, with cases being reported in every district, among unvaccinated, partially vaccinated and fully vaccinated children.

The RGCB findings also challenge the assumption that high vaccination coverage alone guarantees adequate population immunity. WHO’s elimination strategy relies on high two-dose coverage, effective surveillance and rapid outbreak response. Further research is needed to understand the reasons for the variable antibody response in vaccinated children.

Keralam now needs a larger epidemiological investigation to determine how many laboratory-confirmed cases occurred in children with two validated doses of the vaccine, in children with partial or no immunization, and by age of those affected. Such evidence would help determine whether current outbreaks are largely due to incomplete coverage or whether other gaps in immunity also need to be addressed.

Published – 8 Nov 2026 06:09 IST