What does 95% COVID-19 vaccine efficacy really mean?
Authors: Piero Olliaro
Journal: The Lancet Infectious Diseases (2021)

It is imperative to dispel any ambiguity about how vaccine efficacy shown in trials translates into protecting individuals and populations. The mRNA-based Pfizer1Polack FP Thomas SJ Kitchin N et al.Safety and efficacy of the BNT162b2 mRNA covid-19 vaccine.N Engl J Med. 2020; 383: 2603-2615Crossref PubMed Scopus (9507) Google Scholar, 2PfizerPfizer-BioNTech COVID-19 vaccine (BNT162, PF-07302048). Vaccines and Related Biological Products Advisory Committee briefing document.https://www.fda.gov/media/144246/downloadDate accessed: January 29, 2021Google Scholar and Moderna3US FDAVaccines and Related Biological Products Advisory Committee meeting.https://www.fda.gov/media/144434/downloadDate: December 17, 2020Date accessed: January 29, 2021Google Scholar vaccines were shown to have 94–95% efficacy in preventing symptomatic COVID-19, calculated as 100 × (1 minus the attack rate with vaccine divided by the attack rate with placebo). It means that in a population such as the one enrolled in the trials, with a cumulated COVID-19 attack rate over a period of 3 months of about 1% without a vaccine, we would expect roughly 0·05% of vaccinated people would get diseased. It does not mean that 95% of people are protected from disease with the vaccine—a general misconception of vaccine protection also found in a Lancet Infectious Diseases Editorial.4The Lancet Infectious DiseasesAn exceptional vaccination policy in exceptional circumstances.Lancet Infect Dis. 2021; 21: 149Summary Full Text Full Text PDF PubMed Scopus (9) Google Scholar In the examples used in the Editorial, those protected are those who would have become diseased with COVID-19 had they not been vaccinated. This distinction is all the more important as, although we know the risk reduction achieved by these vaccines under trial conditions, we do not know whether and how it could vary if the vaccines were deployed on populations with different exposures, transmission levels, and attack rates. Simple mathematics helps. If we vaccinated a population of 100 000 and protected 95% of them, that would leave 5000 individuals diseased over 3 months, which is almost the current overall COVID-19 case rate in the UK. Rather, a 95% vaccine efficacy means that instead of 1000 COVID-19 cases in a population of 100 000 without vaccine (from the placebo arm of the abovementioned trials, approximately 1% would be ill with COVID-19 and 99% would not) we would expect 50 cases (99·95% of the population is disease-free, at least for 3 months). Accurate description of effects is not hair-splitting; it is much-needed exactness to avoid adding confusion to an extraordinarily complicated and tense scientific and societal debate around COVID-19 vaccines. I declare no competing interests. An exceptional vaccination policy in exceptional circumstancesJust as hope for relief from the COVID-19 pandemic brightened in December, with emergency use approval for vaccines following phase 3 trials and the start of immunisation programmes, unprecedented numbers of cases, hospital admissions, and deaths have been recorded, including in countries such as Germany and Japan that were previously celebrated for their public health response. Predictions that the northern hemisphere winter would be a difficult time for control of COVID-19, as is typically the case with respiratory viral diseases, have proved to be correct. Full-Text PDF