The fourth vaccine shot is powerful in minimizing COVID symptoms but doesn’t provide much stronger protection against catching the virus than a three-dose course, new Israeli research suggests.
Three months after injection, effectiveness in avoiding “substantial symptomatic disease” was 71% for the Pfizer shot and 89% for the Moderna shot. Researchers defined “substantial” as involving two days or more in bed, and the percentage referred to added protection from the fourth shot.
For the Moderna vaccine, 20% of the three-shot participants had substantial symptomatic disease but only 3% of the four-shot participants did. For the Pfizer vaccine, 23% of the three-shot participants had substantial symptomatic disease but only 9% of the four-shot participants did.
The research, newly peer-reviewed and published in Nature Communications, was based on monitoring 776 healthcare workers at Sheba Medical Center in early 2022, at the height of Israel’s Omicron wave.
The new vaccines that have been tailored to fight newer variants weren’t yet available, so the results could be different if the study were conducted today.
The study found that protection against infection was just 3 percent for the fourth Pfizer shot, compared to someone who has received three doses, and 25% for the fourth dose of the Moderna vaccine compared to three. Pfizer is the main vaccine supplier in Israel.
Vaccine effectiveness figures in this study don’t refer to the overall level of protection that someone with four shots has, but instead is a measure of the extra protection delivered by the fourth shot, compared to their initial three.
The rates were calculated by comparing infection and disease rates between fourth-shot recipients and those who only had three shots.
Some 40% to 50% of the study population were infected, with only a small difference depending on whether people had three or four shots.
“Our study did not show that the fourth vaccine dose was effective in protecting against infection, compared to receiving three vaccine doses,” wrote lead researcher Prof. Gili Regev-Yochay, head of infectious diseases at Sheba, and her colleagues.
“Indeed, nearly half of the study population were infected with SARS-CoV-2, despite receiving three or four vaccine doses.
“Yet, we show high vaccine effectiveness against substantial symptomatic disease, defined as disease leading to spending at least two days in bed feeling unwell.”
They wrote that their study goes further than previous research, which highlighted the value of fourth doses in preventing severe illness, as it points to the “high effectiveness of a fourth dose even in non-severe but substantially symptomatic disease.”
Experts say that while vaccines initially generated excitement for almost entirely blocking infection, the main objective was always to minimize serious disease, and therefore research like Sheba’s is still positive.
“The main thing we can learn here is that the vaccine is working,” epidemiologist Prof. Manfred Green, who wasn’t involved in the Sheba study, told The Times of Israel.
“It’s optimistic as it shows that the vaccine is preventing significant disease, including from Omicron. The fact that people don’t tend to get very sick if they have received a fourth shot is important.”
He said that fourth shots — and any subsequent boosters — are particularly important for the elderly and at-risk, who are more vulnerable from serious COVID illness. “Fourth shots are shown to be an important part of our efforts to make the pandemic manageable,” he commented.