You’ve probably heard that Pfizer’s COVID-19 vaccine efficacy is 95%, Moderna’s 94% and Johnson & Johnson 66%. But what do these numbers actually mean?
This is not just an academic question. The way people understand these numbers affects the way they think about the vaccine, whether they get it and how they act after receiving it, all of which have far-reaching consequences for the pandemic.
How then 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 it’s an extremely effective vaccine,” said Brianne Barker, a virologist at Drew University in New Jersey, referring to the Pfizer vaccine. “It’s much more effective than you’d think.”
One common misconception is that 95% efficacy means that 5% of those vaccinated in the Pfizer clinical trial received COVID. But it is not true; the actual percentage of vaccines in the Pfizer (and Moderna) trials that COVID-19 received was about one 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 participants in the control group, who were not vaccinated. In other words, vaccinated people in the Pfizer clinical trial were 20 times less likely to receive COVID-19 than the control group.
That makes the vaccine ‘one of the most effective vaccines we have,’ Barker told WordsSideKick. By comparison, the measles, mumps and rubella (MMR) vaccine is 97% effective against measles and 88% effective 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 the year’s vaccine and flu strains), but it still has an estimated 7.5 million flu cases in the US during the 2019-2020 flu season, According to the CDC.
So, if efficiency means a percentage fewer COVID-19 cases, then what counts as a ‘COVID case’? Both Pfizer and Moderna have defined a case as at least one symptom (however mild) and a positive COVID-19 test. Johnson and 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 by this definition may be mildly affected or disabled and feel quite ill for several weeks.
Barker warns that it is difficult to directly compare the efficacy between the vaccines Johnson & Johnson, Pfizer and Moderna, because the clinical trials took place in different geographical areas with different populations, and at slightly different times in the pandemic when different variants of COVID -19 occurred. was in circulation. ‘There were more people who had the B117 [U.K. variant] or other types of variants during the Johnson & Johnson trial than during the Moderna trial, ”she said.
And none of the three vaccination trials looked at asymptomatic COVID-19 at all. ‘All these effectiveness numbers are protection against symptoms, not protection against infection,“Barker said. (Some early studies suggest that the vaccines Pfizer and Moderna also reduce the number of viral particles in a person’s body, called viral charge, and the likelihood that it will test positive at all, which will reduce transmission. .Because, because we do not), do not yet know that people “can not throw away their mask” once they have been vaccinated, Barker said.
But all three trials also used a second, possibly more important, definition of ‘cases’. What we care about most is protecting people from the worst effects of COVID-19: hospitalization and death. Thus, Moderna, Pfizer and Johnson & Johnson also measured how their vaccines perform against serious diseases (meaning that the heart or respiratory rate is severely affected, the need for supplementary oxygen, ICU admission, respiratory failure or death).
All three vaccines were 100% effective six weeks after the first dose (for Moderna) or seven weeks after the first dose to prevent serious diseases (for Pfizer and Johnson & Johnson, of which the latter requires only one dose). Zero vaccines in any of the trials were hospitalized or died of COVID-19 after the vaccines were fully effective.
“We are incredibly happy with how effective these vaccines have been,” Barker said.
Originally published on Live Science.