You’ve probably heard that Pfizer’s COVID-19 vaccine is 95% effective, Moderna’s 94%, and Johnson & Johnson’s 66%. But what do these numbers actually mean?
It’s not just an academic question. How people understand these numbers affects how they think about the vaccine, whether they get it and how they behave after they get it, all of which has implications for the larger-scale pandemic.
So how should people interpret these numbers?
Related: Quick Guide: COVID-19 Vaccines and How They Work
“I think it’s important for people to understand that this is an extremely effective vaccine,” said Brianne Barker, a virologist at Drew University in New Jersey, referring to the Pfizer vaccine. “This is much more effective than you may think.”
A common misunderstanding is that 95% efficacy means that in the Pfizer clinical trial, 5% of vaccinated people received COVID. But that is not true; the actual percentage of vaccinated people in the Pfizer (and Moderna) trials who received COVID-19 was about a hundred times less than that: 0.04%.
What the 95% actually means is that vaccinated people had a 95% lower risk of getting COVID-19 compared to the control group participants, who were not vaccinated. In other words, vaccinated people in the Pfizer clinical trial were 20 times less likely than the control group to get COVID-19.
That makes the vaccine “one of the most effective vaccines we have,” Barker told Live Science. In comparison, the two-dose measles, mumps, and rubella (MMR) vaccine is 97% effective against measles and 88% against mumps, according to the Centers for Disease Control and Prevention (CDC). The seasonal flu vaccine is between 40% and 60% effective (it varies from year to year depending on that year’s vaccine and flu strains), but it still prevented an estimated 7.5 million cases of flu in the US during flu season 2019-2020, According to the CDC.
So if the efficacy means a few percent fewer cases of COVID-19, what counts as a “case of COVID”? Both Pfizer and Moderna defined a case with at least one symptom (however mild) and a positive COVID-19 test. Johnson & Johnson defined a ‘case’ as a positive COVID-19 test plus at least one moderate symptom (such as shortness of breath, abnormal blood oxygen levels, or abnormal breathing rate) or at least two milder symptoms (such as fever, cough, fatigue, headache, or nausea). Someone with a moderate case of COVID-19 according to this definition may be either mildly affected or incapacitated for work and feel quite ill for a few weeks.
Barker cautions that directly comparing efficacy between the Johnson & Johnson, Pfizer and Moderna vaccines is difficult because the clinical trials were in different geographic areas with different populations and at slightly different times in the pandemic when different variants of COVID-19 were circulating. “There were more people who had the B117 [U.K. variant] or different kinds of variants during the time of the Johnson & Johnson process than during the Moderna process, ”she said.
And none of the three vaccine studies looked at asymptomatic COVID-19 at all. “All of these efficacy figures are protection against symptoms, not protection against infection,Barker said. (Some early research suggests that the Pfizer and Moderna vaccines also reduce the number of viral particles in a person’s body, called viral load, and the likelihood of testing positive at all, which would reduce transmission.Don’t know yet that people “can’t throw their mask away” once they are vaccinated, Barker said.)
But all three studies also used a second, possibly more important, definition of “cases.” What we care about most is protecting people from the worst consequences of COVID-19: hospitalization and death. So Moderna, Pfizer and Johnson & Johnson also measured how their vaccines performed against serious diseases (meaning that the heart or respiratory rate was severely affected, the need for additional oxygen, IC admission, respiratory failure, or death).
All three vaccines were 100% effective in preventing serious illness six weeks after the first dose (for Moderna) or seven weeks after the first dose (for Pfizer and Johnson & Johnson, the latter of which requires only one dose). Zero vaccinated people in any of the studies were hospitalized or died from COVID-19 after the vaccines took full effect.
“We are incredibly lucky with how effective these vaccines are,” said Barker.
Originally published on Live Science.