Impact of increased influenza vaccination in 2-3-year-old children on disease burden within the general population: A Bayesian model-based approach

Abstract

Introduction: During the 2013-2014 influenza season, Public Health England extended routine influenza vaccination to all 2- and 3-year-old children in England. To estimate the impact of this change in policy on influenza-related morbidity and mortality, we developed a disease transmission and surveillance model informed by real-world data.

Methods: We combined real-world and literature data sources to construct a model of influenza transmission and surveillance in England. Data were obtained for four influenza seasons, starting with the 2010-2011 season. Bayesian inference was used to estimate model parameters on a season-by-season basis to assess the impact of targeting 2- and 3-year-old children for influenza vaccination. This provided the basis for the construction of counterfactual scenarios comparing vaccination rates of ~2% and ~35% in the 2- and 3- year-old population to estimate reductions in general practitioner (GP) influenza-like-illness (ILI) consultations, respiratory hospitalizations and deaths in the overall population.

Results: Our model was able to replicate the main patterns of influenza across the four seasons as observed through laboratory surveillance data. Targeting 2- and 3-year-old children for influenza vaccination resulted in reductions in the general population of between 6.2-9.9% in influenza-attributable GP ILI consultations, 6.1-10.7% in influenza-attributable respiratory hospitalizations, and 5.7-9.4% in influenza-attributable deaths. The decrease in influenza-attributable ILI consultations represents a reduction of between 4.5% and 7.3% across all ILI consultations. The reduction in influenza-attributable respiratory hospitalizations represents a reduction of between 1.2% and 2.3% across all respiratory hospitalizations. Reductions in influenza-attributable respiratory deaths represent a reduction of between 0.9% and 2.4% in overall respiratory deaths.

Conclusion: This study has provided evidence that extending routine influenza vaccination to all healthy children aged 2 and 3 years old leads to benefits in terms of reduced utilization of healthcare resources and fewer respiratory health outcomes and deaths.

Fig 1. Influenza-like-illness consultation rates (CPRD and RCGP), all respiratory consultation rates (CPRD), respiratory hospitalization (HES), and respiratory deaths (ONS) data inputs for each season of the model.

CRPD: Clinical Practice Research Datalink; RCGP: Royal College of General Practitioner; HES: Hospital Episode Statistics; ONS: Office for National Statistics.

Fig 2. Matrix of age-specific contacts as derived from the POLYMOD study.

Fig 3. Structure of the inference model.

^aARTI: acute respiratory tract infections; CPRD: Clinical Practice Research Datalink; HES: Hospital Episode Statistics; ILI: influenza-like-illness; ONS: Office for National Statistics; RCGP: Royal College of General Practitioners.

Fig 4. Schematic flow of patients between compartments (S-Susceptible, E-Exposed, I-Infecting, R-Resistant) for a single age group of the model.

In model equations E and I compartments are further split into two compartments each.

Fig 5. Surveillance pyramid of influenza: A schematic breakdown of influenza-infected population into groups.

CPRD: Clinical Practice Research Datalink; RCGP: Royal College of General Practitioners.

Fig 6. Model-estimated number of weekly influenza-like-illness (ILI) consultations (grey line) vs Clinical Practice Research Datalink observed number of weekly ILI consultations (blue line).

Fig 7. Model-estimated number of weekly respiratory hospitalizations (grey line) vs observed number of weekly respiratory hospitalizations (blue line).

Fig 8. Model-estimated number of weekly respiratory deaths (grey line) vs observed number of weekly respiratory deaths (blue line).

Fig 9. Strain-specific weekly infections for each season, by age group.

Fig 10. Comparison of rates of influenza infection and influenza-attributable burden between observed (Scenario 1) and modelled (Scenario 2) values.

Horizontal bar is the median, with shaded bar (“hinges”) representing 25th and 75th percentiles. Vertical bar spans values within 1.5 times inter-quantile range from hinges. In seasons 2010–2013 the modelled rates are with targeted vaccination. In 2013–2014 lack of targeted vaccination is the model.