Family Physician Nudges for Influenza Vaccination Among Older Adults

Influenza vaccination is the most effective measure for preventing influenza. However, the influenza vaccination rate in China is extremely low, with older adults, a high-risk group for influenza, having a vaccination rate of only about 4%. Beginning in 2026, China provides free influenza vaccination for adults aged 65 years and older in rural areas, and allows the personal accounts of employees' basic medical insurance to be used to pay for influenza vaccination for the account holders and their family members, thereby removing the economic barrier to vaccination. Under these payment policies, how to design effective behavioral intervention strategies to increase the vaccination rate has become an important issue.

This study proposes to conduct three independent, parallel cluster randomized controlled trials to implement a family-physician influenza vaccination nudge intervention among adults aged 55 years and older, and to evaluate the effect of the intervention on improving the influenza vaccination rate among older adults under three distinct payment policies. The three sub-trials are conducted under the free vaccination policy for adults aged 65 years and older in rural areas, the self-paid vaccination policy for adults aged 55-64 years in rural areas, and the partially insurance-covered vaccination policy for adults aged 55 years and older in urban areas, respectively. A health economic evaluation will be conducted concurrently, together with a feasibility evaluation based on semi-structured interviews, so as to empower family physicians to promote vaccination among older populations.

Sample sizes are calculated independently for the three sub-trials, based on the primary outcome measure (influenza vaccination uptake) and the main analysis method (comparison of rates between two groups). For the rural sub-trial (free-vaccine policy), taking 10% vaccination rate as the control-group baseline and assuming the nudge intervention raises the rate to 18% (an 8-percentage-point absolute increase), with α = 0.05 (two-sided) and ICC = 0.02, an average of 20 participants per cluster and 54 clusters (family physicians) are required to achieve the power of 90%, resulting in a total sample of 1,080 adults aged 65 years and older. For the rural sub-trial (out-of-pocket policy), taking 4% vaccination rate as the control-group baseline and assuming the intervention raises the rate to 8% (a 4-percentage-point increase, attenuated by economic factors), with the same α and ICC, and an average of 43 participants per cluster and 54 clusters (family physicians) are required to achieve the power of 85%, resulting in a total sample of 2,322 adults aged 55-64 years. For the urban sub-trial (out-of-pocket/insurance policy), taking 5% vaccination rate as the control-group baseline and assuming the intervention raises the rate to 10% (a 5-percentage-point increase, attenuated by economic factors), with the same α and ICC, and an average of 44 participants per cluster and 42 clusters (family physicians) are required to achieve the power of 85%, resulting in a total sample of 1,848 adults aged 55 years and older.

A cluster randomized controlled design will be employed across three representative regions in China. Taking the level of economic development into account, three provinces are selected from the eastern, central, and western regions, and in each province, 3-4 community health service centers (CHCs) are selected in urban and rural areas respectively. In total, 9-12 urban/rural CHCs are recruited, yielding approximately 42 urban and 54 rural family physician clusters. Older adults will be recruited during the primary care visits at the participating CHCs, where all eligible patients meeting inclusion criteria are invited to participate and provide informed consent.

Randomization is performed independently for each sub-trial, with the family physician as the cluster unit. The randomization sequence will be generated in advance by an independent statistician using a computer-generated random number method. Owing to the nature of the intervention, both family physicians and participants are aware of group assignment, and blinding cannot be fully implemented; however, local coordinators, outcome assessors, data managers, and statistical analysts are blinded to group allocation.

The intervention group receives a three-component nudge: (1) an endowment-framing educational pamphlet and verbal recommendation for influenza vaccination by the family physician; (2) on-site default appointment (opt-out) for vaccination; and (3) a telephone reminder at two weeks for those not yet vaccinated. The control group maintains the existing routine service model, with only standard influenza vaccination health education provided during routine encounters.

The intention-to-treat (ITT) approach is used as the primary analysis strategy. Influenza vaccination status at 1 month and 3 months is analyzed using generalized estimating equations (GEE) with the family physician as the clustering unit and the community health service center as a fixed effect, adjusting for prespecified covariates (region, sex, age, marital status, type of medical insurance in the urban sub-trial, and prior influenza vaccination history). Adjusted relative risk and absolute risk difference with 95% confidence intervals are reported. Time-to-vaccination is evaluated using Kaplan-Meier curves and Cox proportional hazards models with robust standard errors. Prespecified subgroup analyses assess heterogeneity of intervention effects. A per-protocol analysis serves as a sensitivity analysis. A health economic evaluation (cost-effectiveness with ICER calculation) and a feasibility evaluation via semi-structured interviews with family physicians and older adults are conducted alongside the trial.