The vaccine effectiveness fallacy

Robert Böhm, RWTH Aachen Universitet, is visiting SODAS and will be presenting at a SODAS Talk Tuesday the 27th of November 2018.


Vaccination provides direct benefits for the vaccinating individuals by reducing the

likelihood of contracting infectious diseases at marginal costs. In addition, most

vaccines provide indirect benefits for unvaccinated individuals by reducing the spread

of pathogens in the population (herd immunity). When considering both its direct and

indirect benefits, individuals have an incentive to free-ride on others’ indirect

protection with increasing vaccination rates. Previous research on interactive

vaccination decisions assumed explicitly or implicitly that vaccinations protect

effectively against contracting diseases, i.e., in 100% of the vaccinations. However,

this is virtually never achieved and the effectiveness of vaccines varies widely.


In the present research, we theoretically analyze the behavioral consequences of

ineffective (vs. effective) vaccines in a game-theoretical framework of interactive

vaccination decisions. We find that ineffective vaccines increase the individual

incentives for vaccination because the indirect benefits of herd immunity decrease. To

investigate the effect of vaccine (in)effectiveness on vaccination behavior empirically,

we conducted two online experiments using convenience samples of the normal

population (N = 358) and health care workers (N = 138). We observe a vaccine

effectiveness fallacy: participants reduce their vaccination intentions with decreasing

vaccine effectiveness. In a second step, we conducted an additional laboratory

experiment (N = 288), implementing an interactive vaccination game with monetary

incentives. In contrast to the surveys, participants received full information about the

direct and indirect effect of vaccinations and the corresponding consequences of

decreasing vaccine effectiveness. Results show that vaccination becomes more likely

when participants face an ineffective (vs. effective) vaccine, eliminating the vaccine

effectiveness fallacy. We discuss the implications for vaccination policy and potential

interventions to increase vaccine uptake.


The SODAS Talk will take place in building 35, 2nd floor, room 36 (35.2.36) of the CSS Campus, Copenhagen University, from 10.30am - 11.30am.

If you have questions or want to know more, please write Agnete Vienberg Hansen at