It depends on what time of the year you were born, according to new Australian data.
Researchers in Western Australia have suggested that scheduling when infants are immunised against the respiratory syncytial virus based on the season of their birth could prevent more than 50 RSV-related hospitalisations per year.
The new findings, based on a statewide study of administrative health data, suggests RSV hospitalisations peak for summer-born infants between five and eight months of age and between eight and 10 months for spring-born infants – beyond the time they are protected by the maternal RSV vaccine.
“These findings are important for determining the optimal protection strategy for preventing RSV-hospitalisations in infants,” the researchers wrote in The Journal of Infectious Diseases.
“While a maternal vaccine strategy providing protection for up to six months of age – with the highest protection from birth to three months of age, as supported by observational effectiveness data – is optimal for infants born at the beginning or during the annual RSV season, it may leave an unprotected window for summer- and spring-born infants.
“As nirsevimab can be administered to infants at any age up to 24 months and provides six months of protection, the optimal strategy to cover the peak ages of RSV-hospitalisation for spring- and summer-born infants could be to offer nirsevimab prior to the onset of the RSV season when summer-born infants approach three months of age and spring-born infants approach five months of age.”
Data from 411,900 births occurring between January 2010 and April 2024 were used, linked to cases of laboratory-confirmed RSV hospital admission data from other administrative health datasets. The distribution of births across each of the four seasons was relatively even: 25.1% in summer, 26.3% in autumn, 24.2% in winter, and 24.4% in spring.
There were 5593 RSV-related hospitalisations over the course of the study. When considering all infants regardless of their season of birth, hospitalisations peaked during the second month of life (32.3/1000 infant-years) before decreasing to 6.2/1000 by the twelfth month of life.
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However, the age-specific incidence rate of RSV hospitalisations – and the timing of peak periods – varied according to which season infants were born in. RSV hospitalisations were highest in autumn- and winter-born infants aged two to three months (55.0/1000) and one to two months (64.2/1000), respectively. In contrast, hospitalisations for summer-born infants peaked during their sixth month of life (24.8/1000) and in the ninth month for spring-born infants (17.3/1000).
“This equates to an annual average of 43 RSV hospitalisations for summer-born infants aged five to eight months, or 47% of all RSV hospitalisations in the first year of life for summer-born infants… [and] an annual average of 27 RSV hospitalisations for spring-born infants aged eight to ten months, or 38% of all RSV hospitalisations for infants born in spring,” wrote the researchers.
“The 2024 WA nirsevimab program achieved 71% coverage among eligible infants and reduced RSV hospitalisations by 57%. Using those figures, we estimate that offering nirsevimab to spring- and summer-born infants in WA could avert an average of 65 additional infant RSV hospitalisations annually.
“With an estimated index RSV hospitalisations cost for infants of AUD$12,643 (standard deviation $21,644), this could translate into saving up to an additional AUD$821,795 in hospital costs per year.”
While acknowledging that their data came from a single state, the researchers felt their findings would generalise beyond Western Australia.
“The overall age distribution of RSV hospitalisations in our population mirrors global data from higher and upper-middle income countries. This recent global analysis by chronological month of age and birth month highlighted that the risk of RSV hospitalisations can vary substantially by month of birth, underscoring the importance of identifying the highest risk months when attempting to design an optimal immunisation program for preventing severe infant RSV,” they wrote.
