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You’ve probably heard that Pfizer’s COVID-19 vaccine is 95% effective, Moderna 94% effective, and Johnson & Johnson 66%. But what do these numbers really mean?
It is not just an academic question. How people understand these numbers affects how they think about the vaccine, if they get it, and how they behave after receiving it, which has implications for the wider pandemic.
So how should people interpret these numbers?
Related: Quick guide: COVID-19 vaccines and how they work
“I think it’s important that people understand that this is an extremely effective vaccine,” said Brianne Barker, a virologist at Drew University in New Jersey, referring to the Pfizer vaccine. “It’s much more efficient than you might think.”
A common misconception is that 95% efficiency means that in the Pfizer clinical trial, 5% of those vaccinated contracted COVID. But this is not true; the actual percentage of people vaccinated in the Pfizer (and Moderna) trials who had COVID-19 was about a hundred times less than that: 0.04%.
What the 95% actually means is that those vaccinated 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 contract 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 (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 40% to 60% effective (varies from year to year, depending on that year’s vaccine and flu strains), but it has still prevented an estimated 7.5 million cases of influenza in the states -United during the 2019-2020 influenza season, According to the CDC.
So if effectiveness means a percentage of fewer COVID-19 cases, what counts as a “COVID case”? Pfizer and Moderna both defined a case as having at least one symptom (even 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 symptoms milder (such as fever, cough, fatigue, headache or nausea). A person with a moderate case of COVID-19 according to this definition could be mildly affected or incapacitated and feel fairly sick for a few weeks.
Barker warns that it is difficult to directly compare the effectiveness of the Johnson & Johnson, Pfizer and Moderna vaccines, as clinical trials have taken place in different geographies with different populations and at slightly different times of 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 variations during the Johnson & Johnson trial than during the Moderna trial, ”she said.
And none of the three vaccine trials looked at asymptomatic COVID-19 at all. “All of these efficacy figures are protection against symptoms, not protection against infection.,(Some preliminary 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 testing positive at all, which would reduce transmission. Yet, because that we don’t know yet that for sure people “can’t throw off their masks” after they’re vaccinated, Barker said.)
But all three trials also used a second, potentially larger, definition of “case.” What matters most to us is protecting people from the worst outcomes of COVID-19: hospitalization and death. Thus, Moderna, Pfizer and Johnson & Johnson also measured the performance of their vaccines against serious disease (which meant severely affected heart or respiratory rate, the need to oxygen, Intensive care 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 requiring only one dose). No one vaccinated in any of the trials were hospitalized or died from COVID-19 after the vaccines took full effect.
“We are extremely fortunate with the effectiveness of these vaccines,” Barker said.
Originally posted on Live Science.
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