No vaccine escape

The measles virus driving Bangladesh’s worst outbreak in recent years has not developed genetic changes that would allow it to evade protection offered by the existing vaccine, according to preliminary findings from an investigation by the International Centre for Diarrhoeal Disease Research, Bangladesh (icddr,b).

The study, released on Sunday, found that the virus circulating in Bangladesh belongs to the globally recognised B3 genotype and that children who had received both recommended doses of the measles vaccine were less likely to test positive than those who were unvaccinated or only partially vaccinated.

The findings also identified infants younger than nine months-who are not yet eligible for routine measles-rubella vaccination-as the most affected age group, highlighting a significant immunity gap during early infancy.

The scale of the outbreak has fuelled public concern that the virus may have mutated enough to reduce vaccine effectiveness.

However, the researchers said their genetic analysis found no evidence to support that claim.

According to the Directorate General of Health Services (DGHS), Bangladesh recorded 122,763 suspected measles cases and 15,272 laboratory-confirmed infections between March 15 and July 25, 2026.

During the same period, 721 suspected measles-related deaths and 95 laboratory-confirmed deaths were reported.

The investigation was carried out jointly by icddr,b and Bangladesh Shishu Hospital and Institute (BSH&I), involving 341 children aged up to 14 years who were admitted to BSH&I with suspected measles between April 20 and May 21. Laboratory testing confirmed measles infection in 324 of the children.

Infants aged between three and eight months accounted for the largest share of confirmed infections, with 137 cases.

Researchers grouped the 341 children according to age and vaccination status. Among 146 infants younger than nine months, who were too young to receive routine vaccination, 142 tested positive for measles, representing 97 per cent.

Of the 126 children who were eligible for vaccination but had received no dose, 122, or 97 per cent, were confirmed with measles.

Among 48 children who had received one vaccine dose, 44, or 92 per cent, tested positive. Of the 21 children who had received both recommended doses, 16, or 76 per cent, were confirmed with measles.

The researchers stressed that this should not be interpreted as meaning that 76 per cent of all fully vaccinated children contract measles.

The figure applies only to the 21 fully vaccinated children who had already been admitted to hospital with suspected measles and were included in the investigation.

They said the steadily lower rate of confirmed infection-from around 97 per cent among children with no vaccine protection to 76 per cent among those fully vaccinated-is consistent with the vaccine providing protection.

However, a larger population-based study involving healthy children would be needed to determine the vaccine’s effectiveness during the outbreak.

“The size of this outbreak has understandably raised concerns about whether the vaccine is still working.

Our findings provide no evidence that the virus has escaped vaccine protection,” said Dr Noorjahan Maliha, Associate Scientist at the Virology Laboratory of icddr,b’s Infectious Diseases Division.

“We observed lower positivity among children who had received both doses, although this hospital-based investigation cannot provide a population-level estimate of vaccine effectiveness,” she added.

Researchers said the confirmation of measles infection in 16 fully vaccinated children warrants further investigation.

Possible explanations include some children failing to develop a sufficiently strong immune response after vaccination, waning antibody protection over time, malnutrition or other underlying health and individual factors.

The current investigation was not designed to determine which of these factors contributed to infection.

As part of the study, icddr,b conducted phylogenetic analysis of 55 whole-genome virus sequences, including 18 sequenced by icddr,b and 37 by the Institute of Epidemiology, Disease Control and Research (IEDCR).

The analysis showed that all virus samples belonged to the B3 subclade and were genetically similar to strains reported in outbreaks in other countries.

Researchers also identified four mutations in regions of the virus’s H protein that are recognised by antibodies.

The H protein enables the virus to enter human cells and is one of the principal targets of vaccine-induced immunity.

Some of these mutations have previously been identified in measles viruses circulating internationally.

However, researchers emphasised that their presence does not indicate the virus has become resistant to the current vaccine.

The findings also underscored a period of heightened vulnerability among young infants.

Under Bangladesh’s routine immunisation schedule, children receive the first dose of the measles-rubella vaccine at nine months and the second at 15 months.

“Vaccination remains the most effective way to prevent measles and reduce the risk of severe illness.

At the same time, we need to understand why this outbreak became so large despite comparatively high reported national vaccination coverage,” said Dr Mustafizur Rahman, Senior Director of the Infectious Diseases Division at icddr,b.

Dr Kamrun Nahar, Assistant Scientist at the Virology Laboratory, said further studies are needed to identify why some children miss vaccination and whether national coverage figures conceal communities with significant immunity gaps.

“Evidence from these studies will help Bangladesh refine its vaccination strategy and better protect children during future outbreaks,” she said.

icddr,b Executive Director Dr Tahmeed Ahmed said the preliminary findings answer some important questions but also demonstrate the need for broader research.

He said icddr,b is preparing a comprehensive research programme to examine why some vaccinated children became infected, the factors contributing to severe illness and deaths, the role of malnutrition in vaccine protection, and when infants lose protective antibodies acquired from their mothers.