No genetic link  between measles and vaccine evasion

 
Preliminary findings from an icddr,b investigation have found no genetic evidence that the measles virus circulating in Bangladesh has escaped the protection provided by the existing vaccine.

The scale of the outbreak has led some people to question whether the measles virus has genetically changed enough to reduce vaccine effectiveness. The findings do not support the belief that the vaccine has become ineffective. Children who had received both recommended doses were less likely to test positive among the suspected cases investigated. Genetic analysis also found that the circulating virus belongs to the B3 genotype, which has been reported in outbreaks elsewhere in the world.

According to the Directorate General of Health Services (DGHS), 122,763 suspected cases and 15,272 laboratory-confirmed cases were reported between 15 March and 25 July 2026. During the same period, 721 suspected and 95 confirmed measles-related deaths were recorded.

Researchers from icddr,b and Bangladesh Shishu Hospital and Institute (BSH&I) investigated 341 children aged up to 14 years who were admitted to BSH&I with suspected measles between 20 April and 21 May 2026. Laboratory testing confirmed measles in 324 children.

Children aged between three and eight months constituted the largest affected group, with 137 confirmed cases.

The researchers divided the 341 children into four groups according to their age and vaccination status. Among 146 infants who were below nine months and therefore too young for routine vaccination, 142 tested positive, representing 97 percent.

Among 126 children who were old enough to be vaccinated but had received no dose, 122 tested positive, or 97 percent. Of the 48 children who had received one dose, 44 tested positive, or 92 percent. Among 21 children who had received both recommended doses, 16 tested positive, or 76 percent. This does not mean that 76 percent of all fully vaccinated children are contracting measles. 

The progressively lower positivity, from approximately 97 percent among children with no vaccine protection to 76 percent among those who had received both doses, is consistent with vaccination providing protection. However, a wider study with unaffected children for comparison is needed to measure how effectively the vaccine is working 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. 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,” said Dr Noorjahan Maliha, Associate Scientist in the Virology Laboratory of the Infectious Diseases Division at icddr,b.

The finding that 16 fully vaccinated children were confirmed with measles requires further investigation. Possible explanations include some children not developing a sufficiently strong immune response after vaccination, antibody protection declining over time, malnutrition or other health and individual factors. The present investigation was not designed to determine which of these factors contributed to infection.

icddr,b conducted phylogenetic analysis, which compares the genetic sequences of viruses to understand how closely related they are and how they may have spread. The analysis covered 55 whole-genome sequences, including 18 samples sequenced by icddr,b and 37 by the Institute of Epidemiology, Disease Control and Research (IEDCR).