Seasonal trends and year-on-year changes in vaccine-preventable respiratory viral infections: Insights from a tertiary care center in Thiruvananthapuram, India (2023–2024)

Dear Editor, In a previous publication in this journal, we analyzed the distribution of vaccine-preventable viral respiratory illnesses reported in 2023 at a tertiary care center in Thiruvananthapuram.[1] That study highlighted the implications of temporal disease patterns on vaccination strategies. We reassessed the distribution of respiratory viral infections in 2024. Our objective was to assess whether there were major year-on-year changes in the incidence and seasonality of vaccine-preventable respiratory viral infections The study was conducted as a retrospective analysis using aggregated, de-identified data from hospital laboratory records. As no individual patient data were collected or analyzed, and the study posed minimal risk, the institutional scientific committee waived the requirement for formal ethical clearance. The inclusion criteria remained consistent with the previous study. We analyzed three respiratory viral infections—influenza, COVID-19, and respiratory syncytial virus (RSV)—among patients presenting with acute respiratory symptoms at KIMSHEALTH, a tertiary care hospital in Thiruvananthapuram, Kerala, India. As in the previous study, diagnostic methods included the Xpert Xpress CoV-2/Flu/RSV test (Cepheid, USA), the BioFire® FilmArray® System (bioMérieux, France), and RT-PCR. Additionally, we incorporated data from the QIAstat-Dx Respiratory Panel Plus (QIAGEN, Germany), introduced at our institute in 2024 as a substitute for BioFire® FilmArray® System. Both platforms are multiplex molecular assays that detect the same respiratory viruses of interest with comparable sensitivity and specificity.[2,3] Therefore, this substitution is unlikely to have significantly influenced overall case detection or the year-on-year trends observed in our study COVID-19 The distribution of COVID-19 cases in 2024 showed a marked decline compared to 2023, with consistently lowercase counts observed across all months [Table 1 and Figure 1]. The peaks observed in 2023, particularly in March (188 cases) and April (826 cases), were not mirrored in 2024, where the corresponding case counts were only 5 and 32, respectively. Similarly, the late-year surge in December 2023 (216 cases) was absent in 2024, with only 3 cases reported during that month. Early months such as January and February also demonstrated a reduction, with 45 and 24 cases in 2023 compared to 12 and 7 in 2024, respectively.Figure 1: Monthly trends of vaccine-preventable respiratory viral infections in 2023 and 2024Table 1: Monthly distribution of vaccine-preventable respiratory viral infections in 2023 and 2024This trend could be attributed to increased population immunity from prior vaccinations and infections, reduced transmissibility or severity of circulating variants, or potential shifts in diagnostic and surveillance practices. Notably, no new vaccines were introduced in the country during this period, suggesting that these reductions were influenced by existing immunity and other non-vaccine-related factors. INFLUENZA In 2023, Influenza A showed distinct peaks in June (147 cases) and November (90 cases). In 2024, the maximum number of cases was observed from June to August, with June (116 cases) and August (117 cases) recording the highest numbers. Early months, such as January and February, displayed similar numbers in both years, with 11 and 6 cases in 2023 compared to 11 and 6 cases in 2024. For Influenza B, 2024 saw an increase in cases compared to 2023, with notable peaks in August (62 cases), September (87 cases), and October (46 cases). In contrast, the peaks in 2023 occurred later in the year, with 29 cases in November and 32 cases in December. Early months of both years had lower numbers, with 1 and 3 cases in January and February 2023 compared to 10 and 1 cases in 2024. This aligns with our 2023 conclusion that most influenza cases occur during the monsoon and post-monsoon seasons. It underscores the importance of scheduling vaccinations during the pre-monsoon months to ensure optimal protection.[4] RSV In 2023, RSV cases showed distinct peaks from September to November, with the highest number recorded in September (30 cases), followed by October (41 cases) and November (23 cases). In 2024, RSV activity remained concentrated in the latter half of the year, but the peak shifted slightly earlier, with August (41 cases) and September (49 cases) recording the highest numbers, followed by October (37 cases). Early months in both years had minimal RSV activity, with no cases reported from January to May in 2023 and very few cases during the same period in 2024. Overall, RSV cases increased in 2024, with a total of 168 cases compared to 114 in 2023, while maintaining a similar seasonal pattern with the majority of cases occurring during the cooler, post-monsoon months. A similar seasonal trend was reported in a recent study from Pune, where increased RSV activity coincided with quarters characterized by higher precipitation and cooler months.[5] Our study has several limitations. First, it was conducted at a single tertiary care center in southern India, which may limit the generalizability of the findings to other geographic regions or healthcare settings. Second, the analysis was based on aggregate monthly case counts, and individual-level demographic or clinical data (e.g., age, comorbidities, vaccination status) were not assessed. CONCLUSION The distribution of vaccine-preventable respiratory viral infections in 2024 demonstrated notable differences compared to 2023. COVID-19 cases declined markedly across all months in 2024, with the absence of large surges observed in the previous year, despite no new vaccines being introduced. Influenza A maintained its seasonal peaks but showed an overall decline in 2024, with the highest activity recorded from June to August. In contrast, Influenza B exhibited a marked increase in activity in 2024, with peaks occurring earlier in the year, from August to October, compared to late peaks in 2023. RSV cases also increased in 2024, with a shift in peak activity to slightly earlier months, from August to October, while maintaining its usual seasonal pattern during the cooler, post-monsoon months. These findings reaffirm that the majority of influenza and RSV cases occur during the monsoon and post-monsoon seasons. They emphasize the need for well-timed vaccination strategies and ongoing surveillance to reduce the burden of respiratory viral infections effectively. Financial support and sponsorship There was no funding involved in this study. Conflict of interest None of the authors have any conflicts of interest.