A vaccine that is 60 per cent effective does not protect 60 per cent of people. It reduces the rate of the outcome by 60 per cent in the vaccinated compared with the unvaccinated. Most of the confusion about vaccine effectiveness comes from that sentence, and most of the rest from not asking which outcome.
Efficacy and effectiveness. In a randomised trial, participants are assigned to vaccine or placebo, so the two groups differ only by the vaccine, and the reduction in disease is the vaccine's efficacy. In the real world, people choose whether to be vaccinated, and those who do differ from those who do not: older, sicker, more likely to seek care, more cautious. Effectiveness is the estimate from that real world, and the whole craft is in removing the differences that are not the vaccine.
The test-negative design. The workhorse of influenza and COVID-19 effectiveness studies. Take everyone who arrives at a clinic or hospital with symptoms and is tested; compare vaccination rates among those who test positive and those who test negative. Because everyone in the study sought care, the comparison is protected against the largest bias — that vaccinated people go to the doctor more — and because the test is the outcome, misclassification is limited. It is not perfect: it assumes the vaccine does not affect the other illnesses that bring people in, and it estimates effectiveness against medically attended illness, not against all infection.
Linked cohorts. Countries with linked records — vaccination registers joined to hospital and death data — can follow whole populations and estimate effectiveness against hospitalisation and death directly, adjusting for age, prior infection and comorbidity. The Nordic countries, the United Kingdom, Israel and Qatar produced most of the pandemic's estimates this way. Residual confounding remains: the healthy vaccinee effect, where people well enough to be vaccinated are also less likely to die of anything.
Which outcome. Effectiveness against infection is lowest and wanes fastest; against symptomatic illness higher; against hospitalisation higher still; against death highest and most durable. A report of effectiveness without an outcome is not a report. Influenza vaccines have typically shown a third to a half against medically attended illness, varying with the match between the vaccine and the circulating strains; the RSV vaccines for older adults showed high first-season effectiveness against hospitalisation, with the durability into later seasons the open question.
Waning and time since dose. Effectiveness measured in the month after vaccination is not the effectiveness six months later. Good studies report by interval since dose; comparisons across studies that do not are comparisons of different things.
Reading an estimate. Design, outcome, population, interval since dose, and the confidence interval — which for a single season's estimate can run from twenty per cent to sixty. A point estimate quoted alone has usually been stripped of the part that mattered.