As countries across the globe gear up for mass coronavirus vaccine campaigns, producers of the drugs have been touting percentages in the 90s in terms of how effective they believe their shot will be. But does that really mean nine out of 10 people who get the jab won't get COVID-19?
Two vaccine makers have said that preliminary results from their late-stage studies suggest their experimental vaccines are strongly protective. Moderna last week said its vaccine appears to be nearly 95% effective. Oxford/AstraZeneca also said its vaccine was over 70% effective but this number could go up to 90%. These came on the heels of Pfizer and BioNTech’s announcement that its shot appeared similarly effective.
Those percentages raised hopes around the world that vaccines could help put an end to the pandemic sometime next year if they continue to show that they prevent disease and are safe. But what do the numbers really mean?
The term "vaccine efficacy" is what scientists use to refer to the reduction of disease incidence among a vaccinated group compared to an unvaccinated group under optimal conditions, according to Dr. Shelly McNeil of the Canadian Center for Vaccinology. Efficacy is determined through clinical trials and that is where the figure 95% for some coronavirus vaccines has come from.
Vaccine effectiveness, however, refers to the ability of the drug to prevent disease in the "real world," McNeil explained in a presentation published by the World Health Organization (WHO).
Although it is possible for efficacy and efficiency to be the same, for most vaccines, the effectiveness in practice is lower than in clinical trials. Inhibiting factors in the general population, such as chronic health problems, can decrease a vaccine's effectiveness.
In short, the broad, early efficacy figures don’t tell the whole story. Scientists also need to understand how well the vaccine protects people in different age groups and demographic categories.
For both vaccines, the interim results were based on people who had COVID-19 symptoms that prompted a virus test. That means we don’t know yet whether someone who’s vaccinated might still get infected – even if they show no symptoms – and spread the virus.
Also unknown is whether the shots will give lasting protection, or whether boosters will be required. Pfizer's vaccine, for example, requires a booster 21 days after the initial dose.
Effectiveness numbers will change as the vaccine studies continue since the early calculations were based on fewer than 100 COVID-19 cases in each study. There was concern that coronavirus vaccines might be only as effective as flu vaccines, which have ranged from 20% to 60% effective in recent years.
Early results, however, provide strong signals that the vaccines could prevent a majority of disease when large groups of people are vaccinated.