October found to be the optimum time to vaccinate against influenza

By Pooja Toshniwal PahariaReviewed by Lily Ramsey, LLMFeb 22 2024

In a recent study published in BMJ, researchers investigated the optimum time to vaccinate young infants against influenza.

Study: Optimal timing of influenza vaccination in young children: population based cohort study. Image Credit: Yuganov Konstantin/Shutterstock.com

Background

The yearly influenza vaccination is critical for lowering morbidity and death from seasonal influenza, although large-scale research on optimal scheduling is lacking.

The Centers for Disease Control and Prevention (CDC) advises vaccines in October or September to optimize vaccine-induced protection. Delays in vaccination might lead to influenza virus exposure without immunity.

Experts advise balancing vaccine-elicited protection throughout the season while avoiding missed chances. Young children are at high risk, but there is little clinical data to recommend vaccine scheduling.

About the study

In the current retrospective observational population-based cohort study, researchers examined influenza vaccination and illness patterns among infants born in periods of influenza vaccine availability.

The study included commercially insured children aged two to five years who were vaccinated against influenza between 2011 and 2018 and remained registered in insurance for one or more influenza seasons between September and May months of the following year.

The primary outcome indicators were the rates of influenza diagnosis among pediatric vaccinees, broken down by birth month.

The researchers limited the study to children born between 1 August and 31 January, as preventative visits might alter the influenza vaccination timing in the children.

The cohort only included children who had received influenza vaccines from August to January months of the following year, as evidenced by insurance claims with the International Classification of Diseases, Ninth Revision (ICD-9), and Current Procedural Terminology (CPT) codes for vaccinations against influenza viruses.

The team used the MarketScan Research database to obtain data on children, brothers, sisters, and their parents, all insured with the same policy. The model variables included the child's age, gender, healthcare utilization, family size, and comorbidities, including past pulmonary illness.

The researchers calculated the influenza risk using the actual vaccination month and variables at the pediatric influenza season level. They also investigated whether children had yearly checkups around birthdays and compared influenza vaccination rates on preventive health visits to those on surrounding days.

The team assessed the vaccination timing by birth month, determining if infants born earlier got vaccinations sooner on average.

They performed multiple analyses to evaluate the association between birth month and influenza risk, examining the influence on superficial injury visits, non-influenza infectious illnesses, conjunctivitis, viral gastroenteritis, and common infectious disorders.

They repeated the original study after giving infants random birth months to see if the results were attributable to chance alone.

Results

The study examined vaccination rates among 819,223 children aged two to five years in the United States.

The results showed that children who had November-December vaccinations were less likely to receive an influenza diagnosis, potentially due to unmeasured variables impacting vaccine timing and influenza risk. Vaccinations are often administered on the days of preventative healthcare visits and throughout the birth months.
October-born children showed disproportionate vaccinations, with an average vaccination timing later than August-born ones but earlier than December-born ones. October showed the lowest percentage of influenza diagnoses, with 2.7% of children vaccinated compared to 3.0% in August.

The general schedule of influenza vaccines remained consistent year after year; however, the peak of influenza diagnoses fluctuated. Most children (37% of all child seasons) received vaccinations in October.

January- or December-born children were less likely to receive vaccinations at their preventative visit since many may been immunized earlier in the autumn.

The researchers found no significant variations in children's demographic and clinical parameters between birth months, including their estimated influenza infection risk. However, the influenza vaccination varied by month of birth.

Vaccination took place on average sooner for children born in earlier months. October-born children received more vaccinations than children born in other months.

Unadjusted influenza diagnostic rates varied by birth month, with the lowest rates occurring in children born in October. In an adjusted study, children born in October were less likely to be diagnosed with influenza than in other birth months.

Falsification studies using random birth months and superficial damage, viral gastroenteritis, or conjunctivitis showed no relevant relationships between birth months.

However, in subgroup evaluations focusing on pediatric individuals with prior pulmonary disease history or those in the upper third quartile of Elixhauser comorbid conditions and MSA areas with high influenza severity, the team observed a higher absolute reduction in influenza infection among October-born children than in other birth months.

Conclusion

Overall, the study findings showed that the birth month is associated with the influenza vaccination timing in young children, particularly those with birthdays in October.

October-born children are more likely to receive vaccines and have fewer influenza diagnoses, supporting the guideline for October vaccination.

Utilizing the month of birth for natural experimentation may determine the ideal influenza vaccination timing, as immunity to influenza decreases during the flu season. US public health measures centered on vaccination in October may provide the most protection during average flu seasons.