You have probably heard that the effectiveness of Pfizer’s COVID-19 vaccine is 95%, that of Moderna is 94% and that of Johnson & Johnson is 66%. But what do these numbers really mean?
It is not just an academic issue. The way people understand these numbers affects how they think about the vaccine, whether they receive it and how they behave after receiving it, all with implications for the pandemic on a larger scale.
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 think.”
A common misconception is that 95% effectiveness means that in the Pfizer clinical trial, 5% of vaccinated people contracted COVID. But that is not true; the actual percentage of people vaccinated in Pfizer (and Moderna) tests who received COVID-19 was about a hundred times less than that: 0.04%.
What the 95% really means is that vaccinated people had a 95% lower risk of contracting COVID-19 compared to participants in the control group, who were not vaccinated. In other words, people vaccinated in the Pfizer clinical trial were 20 times less likely than the control group to receive COVID-19.
This makes the vaccine “one of the most effective vaccines we have,” Barker told Live Science. For comparison, the two-dose measles, mumps and rubella vaccine (MMR) is 97% effective against measles and 88% effective against mumps, according to the Centers for Disease Control and Prevention (CDC). THE seasonal flu vaccine has between 40% and 60% effectiveness (varies from year to year, depending on the vaccine and flu strains of that year), but still prevented about 7.5 million cases of flu in the U.S. during the 2019-2020 flu season , According to the CDC.
So, if effectiveness means a few percent fewer cases of COVID-19, what counts as a “case of COVID”? Both Pfizer and Moderna defined a case as having at least one symptom (albeit mild) and a positive COVID-19 test. Johnson & Johnson defined a “case” as having a positive COVID-19 test plus at least one moderate symptom (such as shortness of breath, abnormal blood oxygen levels or abnormal respiratory 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 slightly affected or disabled and feel very bad for a few weeks.
Barker warns that it is difficult to directly compare the efficacy between Johnson & Johnson, Pfizer and Moderna vaccines, because clinical trials took place in different geographic areas with different populations and at slightly different points in the pandemic when different variants of COVID-19 were circulating . “There were more people who had the B117 [U.K. variant] or other types of variants during the Johnson & Johnson test period than during the Moderna test, ”she said.
And none of the three vaccine trials looked at asymptomatic COVID-19. “All of these efficacy numbers are protection against symptoms, not protection against infection,“Barker said. (Some previous studies suggest that the Pfizer and Moderna vaccines also reduce the number of viral particles in a person’s body, called a viral load, and the likelihood of a positive test on everything, which would cut transmission. Still , because we use it I still don’t know for sure, people “can’t throw away their masks” after being vaccinated, said Barker.)
But all three studies also used a second, potentially more important definition of “cases”. What worries us most is protecting people from the worst results of COVID-19: hospitalization and death. Thus Moderna, Pfizer and Johnson & Johnson also measured the performance of their vaccines against serious illnesses (which meant a severely affected heart or respiratory rate, the need for supplements oxygen, ICU admission, respiratory failure or death).
All three vaccines were 100% effective in preventing serious illnesses six weeks after the first dose (for Moderna) or seven weeks after the first dose (for Pfizer and Johnson & Johnson, the last of which requires only one dose). No person vaccinated in any of the trials was hospitalized or died of COVID-19 after the vaccines were fully effective.
“We are very lucky with the effectiveness of these vaccines,” said Barker.
Originally published on Live Science.