There has been a flurry of press releases on vaccine effectiveness. There is a risk of misunderstanding these numbers by lay people. Multiple factors influence the risk of developing Covid-19 illness. These include innate immunity (first defense response when body is exposed to any foreign organism), potential cross-immunity from prior exposure to other corona viruses that cause common cold, quantum of virus exposure, whether a person is following containment measures like face masking, hand hygiene and social distancing, age, gender, presence of other comorbidities and the degree of immune response mounted by a person.
It is also important to understand whether the vaccine is effective in preventing development of illness, reducing duration of hospitalization, need for ventilation, or death. For example, chloroquine was demonstrated not effective in either preventing infection or reducing severity of illness. Remedesivir was effective in reducing only duration of hospitalization and was not effective in preventing progression to ventilation or death. Whereas steroid drugs were the only ones effective in reducing deaths in patients with falling oxygen levels.
To demonstrate effectiveness, vaccines are compared with a placebo (a formulation that is inert but looks like the vaccine). For example, in a study vaccine is given to 20,000 healthy people and placebo is given to another 20,000 healthy people. All people are followed up meticulously. Some of these people may develop Covid-19 over a period of time. Supposing 100 (0.5%) people who received placebo and 10 (0.05%) people who received vaccine develop Covid-19 illness, it implies that vaccine might have prevented Covid-19 illness in 90 more people out of 100 at risk of developing, translating to 90% effectiveness.
Another caveat is that people in both groups should compare well in terms of other variables that can influence development of illness. These include gender, age, presence of comorbidities, socio-economic status, living conditions, face masking, geography etc. Randomization methods adopted by all vaccine studies will, in general, ensure comparability of both the groups.
While effectiveness is one of the measures, how long the protection lasts is yet to be answered. Whether the duration varies with type of vaccine – mRNA, virus vector (human or simian adenovirus), inactivated virus, component, attenuated vaccine – is also not yet known. How each vaccine behaves when the virus mutates will also vary depending upon the type of vaccine.
Vaccines also differ in their cost, route of administration, number of doses, and temperature stability.