The preliminary findings of an icddr,b investigation have found no proof of genetic changes in measles viruses in the country that could reduce vaccine protection.
When 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, icddr,b said in a press release today.
Genetic analysis also found that the circulating virus belongs to the B3 genotype, which has been reported in outbreaks elsewhere in the world.
Researchers from icddr,b and Bangladesh Shishu Hospital and Institute (BSH&I) investigated 341 children aged up to 14 years who were admitted to the hospital with suspected measles between April 20 and May 21. 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. All the children in the investigation had already been admitted to hospital with suspected measles. The percentage refers only to the 21 fully vaccinated suspected patients included in this hospital investigation.
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,” said Noorjahan Maliha, associate scientist in the Virology Laboratory of the Infectious Diseases Division at icddr,b.
“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.
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 findings also highlight a period of vulnerability among young infants. In Bangladesh, the first routine measles-rubella vaccine is provided at nine months and the second at 15 months. Before receiving the first dose, infants depend partly on antibodies transferred from their mothers and on community protection created through widespread vaccination.
An initial analysis of stored samples from 16 mother-child pairs in an earlier icddr,b birth cohort found that maternal measles antibodies declined rapidly during the first months of life.
No protective antibodies were detected at three to eight months of age among the small group of children assessed. As this analysis included only 16 children, further investigation using samples from the larger birth cohort is needed.
“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 Mustafizur Rahman, senior director of the Infectious Diseases Division at icddr,b.
“icddr,b is developing a wider research response to examine why some vaccinated children became infected, what factors are contributing to severe disease and death, how malnutrition may affect protection and when infants lose antibodies received from their mothers,” said Tahmeed Ahmed, executive director of icddr,b.
The researchers emphasised that the parents should not delay or avoid vaccination. Children should receive all measles-rubella vaccine doses for which they are eligible, and parents and caregivers should follow the government’s vaccination guidance during the outbreak.
