Confusion is spreading online as countries report increasing numbers of new COVID-19 cases among people who have been vaccinated.
Recent reports from Israel show that a large portion of cases there have been among individuals who were fully vaccinated.
And last week, the US Centers of Disease Control (CDC) reported that “approximately three-quarters of cases (in a recent outbreak in Massachusetts had) occurred in fully vaccinated persons”.
The reports have led to suggestions that people who are fully vaccinated appear more likely to catch COVID-19 than those who aren’t.
In an interview with US network Fox News, Republican senator Ron Johnson questioned whether the Pfizer vaccine was effective against the highly contagious Delta strain of the virus saying that “84 per cent” of cases in Israel were among vaccinated individuals.
Others, meanwhile, took to social media, with one Twitter user responding to the Massachusetts outbreak by asking: “How the f*** are vaccinated people the majority of the infected?”
But those comments ignore the fact that regions such as Israel and Massachusetts have vaccinated the majority of their populations — 62 per cent and 64 per cent respectively. It stands to reason, therefore, that new cases are more likely to occur in vaccinated communities as they make up more of the population.
As the Washington Post pointed out: “The latest numbers show that 85 per cent of Israeli adults are vaccinated, meaning there are more than five times as many of them as unvaccinated people.”
According to Lucy D’Agostino McGowan, an assistant professor of statistics at Wake Forest University, expressing vaccinated people as a percentage of COVID-19 cases means that “the better the vaccine uptake the scarier this number will seem”.
“This is the wrong metric to look at,” she wrote in a blog post, explaining that it excludes the wider population and, therefore, represents the “probability of being vaccinated given you are sick” rather than “the probability of being sick given you are vaccinated”.
In her blog post Professor McGowan outlined a situation in which 90 per cent of a group of 20 people were vaccinated. Four people in the group contracted COVID-19, including two who were unvaccinated and two who were, which means that 50 per cent of the infections were among vaccinated people.
By another measure, however, the same scenario showed that 100 per cent of unvaccinated people got sick, compared to just 11 per cent of vaccinated people.
In this case, the vaccine would be seen to be 89 per cent effective.
It is also worth noting that so-called “breakthrough” cases (infections in people previously vaccinated) are likely to be less severe. In the recent Massachusetts outbreak, as reported by the CDC, five COVID-19 patients were hospitalised: four had been fully vaccinated, and no deaths were reported.
Similarly, while daily cases continue to rise in Israel, the percentage of COVID-19 patients who become critically ill has fallen to 1.6 per cent compared with 4 per cent during a similar spike prior to vaccines becoming available.
CNN broke down the CDC’s statistics which included 6,587 breakthrough cases as of July 26. Dividing these by the vaccinated population — 163 million at the time — the broadcaster concluded that less than 0.004 per cent of fully vaccinated people experienced a breakthrough case that led to hospitalisation, with less than 0.001 per cent dying from COVID-19.
As Reported by ABC Fact Check