Infants become susceptible to measles earlier than thought

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In countries that have eliminated or nearly eliminated measles, maternal antibodies against the virus have been found to protect infants for only first three months after birth and not six months as is commonly believed. This leaves infants vulnerable to infection for about nine months before the first dose of measles vaccine can be given.

Contrary to common notion that maternal antibodies against measles protect infants for the first six months of age, a small study involving 25 infants once again shows that antibodies from the mother almost disappear by the end of three months. Hence, infants become susceptible to measles infection at the end of three months and not six months as earlier thought.

Currently, as per the World Health Organization (WHO) recommendation, children in countries like India with ongoing transmission of measles are vaccinated only at 9-12 months of age. In countries with no ongoing transmission, the first dose is administered when the baby is 12-15 months of age. This would mean that babies would remain susceptible to measles infection for a longer period of time before they get vaccinated with the first dose.

The study was carried out on 25 infants in a single tertiary hospital in Toronto, Canada. All the 25 infants had no underlying medical condition and were born at 37 weeks or more of gestation to mothers who were on average 32 years old.

Measles is a highly infectious acute viral illness that can lead to severe complications, such as pneumonia, encephalitis, and death. Children younger than five years who get infected with measles have higher rates of complications, hospitalisation, and even death.

Decline of maternal antibodies

The study published in the journal Pediatrics found that of the 25 infants studied, 20% (five of 25) infants had antibodies below the protective threshold even by the end of the first month after birth. And 92% infants had maternal antibodies below the protective threshold by the end of three months. By six months all the babies had antibody levels below the protective threshold.

Based on statistical modelling, the authors found that in Canada the probability of infants getting infected increased with maternal age. This is because pregnant mothers remain protected through vaccination and not through natural infection. They predict that one-month-old infant has 25% probability of getting infected with measles if the mother is 25 years old but the probability increases to 40% if the mother is 40 years old.

Babies are considered to be protected against measles through maternal antibodies for the first six months based on studies carried out in measles-endemic settings. In these settings, like in the case of India, many mothers gain immunity through natural infection and are also continually exposed to the virus, leading to “repeated immunologic boosting and more robust antibody levels”. But in countries where measles has been eliminated or close to elimination, mothers gain immunity through childhood vaccination, which has been associated with lower antibody levels compared with natural infection.

Infants born to mothers in countries where measles virus has been eliminated have lower maternal antibodies and these antibodies quickly fall below the threshold of protection leaving them susceptible to infection before they receive the first measles vaccination dose, the authors note.

How vulnerable are infants in India

However, in the case of India, the virus is in circulation and causes many infections each year (there were nearly 72,000 cases in India during October 2018-2019, the third highest in the world). Also, the vaccine coverage with two doses is less than 95% to offer protection to infants who are yet to be vaccinated. Hence, infants in India who are too young to receive the first dose of the vaccine are at greater risk of getting infected.

The authors note that pregnant mothers cannot be administered measles vaccine as the vaccine uses live, weakened virus. According to an Opinion piece published alongside the paper, vaccine-induced protection would be less in infants if the vaccine is given earlier than recommended. Also, early vaccination may “alter response” after the second dose of vaccine, leading to “lower levels of the antibody” compared with children who are vaccinated as per schedule.

“Therefore, the best strategy for protecting infants against measles is adequate community protection delivered through high coverage [over 95%] of two doses of measles-containing vaccine,” the authors of the paper write.

Published in The Hindu on November 23, 2019

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