Keralam measles outbreaks call for a relook at disease epidemiology

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

Once predominantly a childhood disease affecting children aged one to five, measles is now being reported across age groups, including among fully vaccinated individuals and infants below nine months, who are not yet eligible for routine measles vaccination.

The last major surge in cases was in 2022-23. As of November 6, the Integrated Disease Surveillance Programme had reported 824 confirmed and 1,681 probable measles cases in the State. Five measles-related deaths have been reported, though only one has been confirmed so far.

Measles vaccination was introduced into the Universal Immunisation Programme (UIP) in 1985, with a single dose at nine months. A second dose, at 18 months, was introduced in 2010 to address immunity gaps following the first dose and support measles elimination.

The nine-month threshold was intended to ensure that maternal antibodies transferred through 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 epidemics, we have seen a good number of infants developing measles before nine months, potentially because mothers of the present generation have lower levels of measles antibodies. We also have a chunk of cases among older adults,” says K.K. Purushothaman, former Professor of Paediatrics at Government Medical College, Thrissur.

He believes the State should consider an additional dose of a measles-containing vaccine under the UIP, either during adolescence or for women before marriage. The State Technical Advisory Group on Immunisation should examine the proposal and commission serological studies among adolescents and young women to assess their protective antibody levels, he says.

The concern is supported by findings elsewhere. Following major measles outbreaks in Mumbai in 2023, when more than 31% of reported cases were among infants below nine months, a seroprevalence study found that only 66.8% of mothers had adequate measles antibodies to protect their infants.

A World Health Organisation (WHO) informal technical consultation report, published in Vaccine in May 2025, acknowledged that women acquiring immunity through vaccination rather than natural infection might transfer lower levels of measles antibodies to their babies. However, experts concluded that evidence was insufficient to change the existing recommendation for the first routine dose at 9-15 months.

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

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

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

Keralam generally reports around 95% coverage for the first dose of the measles-rubella (MR) vaccine. Coverage of the second dose, administered at 18 months, has consistently been lower, at around 85-88%, with some districts reporting still poorer coverage.

Incomplete immunisation remains a major driver of outbreaks. The Health department has often attributed outbreaks to districts such as Malappuram and Kozhikode, where vaccination gaps are substantial. However, experts point out that pockets of under-immunisation exist across the State and cases are 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. The WHO’s elimination strategy depends on high two-dose coverage, effective surveillance and rapid outbreak response. Further research is needed to understand the reasons for variable antibody responses among vaccinated children.

Keralam now needs larger epidemiological investigations to establish how many laboratory-confirmed cases occurred among children with two verified vaccine doses, those with partial or no immunisation and the age-break-up of those affected. Such evidence would help determine whether current outbreaks are driven predominantly by incomplete coverage or whether additional immunity gaps also need to be addressed.