PEOPLE WHO’VE BEEN VACCINATED MAY TAKE MORE HEALTH RISKS: Evidence from a flu vaccination campaign

12 Apr 2021

New research finds evidence of increased risk-taking behaviour after flu vaccination, which can counter the vaccine’s effectiveness. The study by Manuel Hoffmann and colleagues concludes that at minimum, policy-makers may want to communicate to individuals that they should not forgo additional protective measures after they get vaccinated, particularly in the current pandemic.

The researchers also find that making it easier for people to get vaccinated – for example, by offering vaccines in the workplace during the working week – improves take-up, as does making use of ‘peer effects’. By lowering opportunity costs of getting vaccinated, it is possible to increase substantially participation in a health campaign; and policies that provide incentives for group behaviour and peer comparisons may be leveraged to improve further people’s willingness to join such campaigns.

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In light of the current pandemic and the vaccination campaigns that are gaining steam globally, it is of the utmost importance to understand the drivers of being vaccinated and how vaccination affects behaviour. Vaccination rates are often lower than recommended.

Therefore, this study asks in a first step: What are the reasons why people get vaccinated? Once people are vaccinated, what is the real world’s effectiveness of being vaccinated? As research in economics shows that the adoption of protective technologies could induce individuals to undertake riskier behaviours, the real-world effectiveness may be lower than the medical effectiveness.

During 2017-18, the research team collaborated with a bank in Ecuador that started a flu vaccination campaign for its employees. Due to capacity constraints, some employees were randomly assigned to get vaccinated during the working week, while others were assigned to the weekend.

The cost of the next best alternative – that is, the opportunity costs of getting vaccinated – were lower during the working week than the weekend. During the working week, employees were already in the bank’s offices, so they could quickly get vaccinated. On the weekend, employees would have to alter their plans to go to the bank’s offices and get immunised. The authors find that vaccination take-up increased by almost 100% for employees assigned to a vaccination appointment during the working week.

They also study whether peer behaviour can affect immunisation coverage. If vaccination provides some protection to other people, free-riding on those benefits may reduce the likelihood to get vaccinated when peers get vaccinated. Alternatively, peer vaccination may improve individual vaccination through the channel of social norms. Therefore, it is unclear whether peer vaccination will increase or decrease the propensity to vaccinate. The study finds that when the proportion of peers getting vaccinated increases by ten percentage points, take-up increase by 6.2 percentage points.

Based on these results, the researchers establish two policy implications for health interventions, be it at the public or company level. First, by lowering opportunity costs it is possible to increase substantially participation in a health campaign. Second, policies that provide incentives for group behaviour and peer comparisons may be leveraged to improve further people’s willingness to join such campaigns.

In a second step, they investigate the real-world effectiveness of the campaign. They exploit the random assignment of employees to the working week to estimate the campaign’s effects on health. They find that employees assigned to the working week, who are more likely to be vaccinated, are equally likely to be sick from influenza-related respiratory diseases and equally likely to take a sick day, just as those who are less likely to be vaccinated.

They then study whether getting vaccinated may trigger behavioural changes. This could be an alternative explanation, adding to medical ineffectiveness, which the evidence does not rule out. In fact, vaccination may make individuals feel protected, and result in risky behaviours that increase the likelihood of respiratory diseases not covered by the flu vaccine.

The results support this idea. The researchers find that employees who are more likely to be vaccinated, are less likely to see a doctor for non-flu respiratory illnesses that share similarities with the flu. Furthermore, they are also less likely to go to the company on-site doctor. Finally, these individuals with a higher probability of being vaccinated report behaviour in an employee survey that may be perceived to threaten health.

Overall, the evidence suggests increased risk-taking behaviour after flu vaccination, which can counter the vaccine’s effectiveness. Therefore, at minimum, policy-makers may want to communicate to individuals that they should not forgo additional protective measures after they get vaccinated. This policy recommendation is particularly relevant in this pandemic.

Manuel Hoffman

Visiting Postdoctoral Scholar | Princeton University | manuelhoffmann@tamu.edu