By Dr. Sanchari Sinha Dutta, Ph.D.

A study conducted at Harvard Medical School in association with Massachusetts General Hospital, USA, finds that influenza vaccination in October has the most protective efficacy against seasonal influenza infection in young children.

The study is published in the British Medical Journal.

Recent guidelines by the US Centers for Disease Control and Prevention (CDC) advise administering influenza vaccination among children and older adults in September or October to maximize its protective efficacy against seasonal influenza virus infection.

Administering the vaccine too early, i.e., in the summer months, may lead to the waning of vaccine efficacy during the upcoming influenza season (September to May). Similarly, delaying vaccination may result in exposure to the circulating influenza virus without gaining vaccine-induced immunity.

The timing of influenza vaccination is particularly vital for young children who are at higher risk of severe infections and related hospitalization. Young children primarily receive vaccines during protective care visits, which are often purposefully scheduled around their birthdays. Thus, a child’s birthday is likely to impact the timing of influenza vaccination.

In this study, scientists have analyzed the rate of influenza diagnosis among vaccinated children to determine optimal vaccination timing.

The study population included commercially insured children aged 2 – 5 years who received influenza vaccination between 2011 and 2018. Only children with birthdays between August 1 and January 31 were included because these children are expected to have their birthday influence on the timing of influenza vaccination.

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The participants’ medical data were collected from the MarketScan Research Database, which contains insurance claims of approximately 30 – 40 million commercially covered Americans. Confounding factors that were considered in the analysis included age, sex, healthcare use, and medical comorbidities of the child, number of beneficiaries on the same policy, and medical comorbidities and healthcare use of family members.  

Data were analyzed to determine the optimal timing of influenza vaccination and its impact on the rate of influenza diagnosis among vaccinated young children.

Important observations

A total of 819,223 children who received influenza vaccination between August and January of a given influenza season were included in the analysis. A similar pattern of vaccination timing and a differential pattern of influenza diagnosis timing was observed in the study.

The analysis of medical data revealed that the majority of children received influenza vaccination in October. The lowest possibility of influenza diagnosis was observed among children who received vaccination in November or December, as compared to those vaccinated in other months.

As mentioned by the scientists, the timing of vaccination may be influenced by many confounding factors. Thus, they could not derive a causal relationship between vaccination timing and influenza diagnosis. Instead, they assessed relationships between birth month, the timing of preventive care visits and vaccination, and the risk of influenza infection.

Birth month and vaccination timing

About 90% of enrolled children who received vaccination during August-January had a preventive care visit during that period. Among children aged 2 years, 3 years, 4 years, and 5 years, about 56%, 59%, 51%, and 41% had a visit in the two weeks around their birth months, respectively. The majority of these children received vaccination on the day of the preventive care visit.

In contrast, a lower probability of receiving the vaccine on the preventive care visit day was observed for children born in December or January. One possible reason could be that many of these children may have received the vaccine earlier in the fall.

A variation in vaccination timing was observed across birth months. The average duration between birth month and vaccination timing was 10 weeks for August-born children, 12 weeks for October-born children, and 14 weeks for December-born children. Children born in October showed a significantly higher probability of being vaccinated in the same month compared to children born in other months.

Birth month and influenza diagnosis

The lowest rate of influenza diagnosis was observed for children born in October. Specifically, the average infection diagnosis rate across influenza seasons was 3% for August-born children, 2.7% for October-born children, and 2.9% for December-born children.