Vaccine freeloaders are persons who do not get vaccinated but who nevertheless reap the benefits of vaccination. The benefits of freeloading arise when the rest of the population is vaccinated and immune; then, the disease does not spread, and so the freeloaders do not contract the disease (Rashid et al, 2012). The desire to reap indirect benefit, however, is not a usual reason for refusing vaccination.

There are many reasons why persons may refuse vaccines, and these reasons are not mutually exclusive. Many vaccines have common adverse effects such as pain and discomfort at the site of administration and short-lasting malaise or fever. Some vaccines have rare but potentially serious adverse effects. Some vaccines are expensive. Getting vaccinated requires time and may entail some inconvenience (Doherty et al, 2016). In some countries, vaccination programs are surrounded by conspiracy theories and are widely shunned (Khan et al, 2020).

Opting out of universal immunization programs is dangerous because if too many persons in the population do so, herd immunity will not develop, and pockets of illness may periodically break out. This has been happening even in developed countries, for example as a result of MMR vaccine refusals (Perrone and Meissner, 2020