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A booster shot of Pfizer’s coronavirus vaccine dramatically lowers the viral load in patients infected with the delta variant, and therefore reduces the risk of transmission, according to a new Israeli study.
The study was carried out jointly by the Technion – Israel Institute of Technology and KSM – the Maccabi Research and Innovation Center. It was published on the MedRxiv website, which is for articles that have not yet been published in a scientific journal.
The researchers concluded that about six months after someone receives the second dose of the vaccine, its effectiveness in reducing the viral load wears off. But a third dose reduces viral loads by a factor of four, restoring the vaccine’s effectiveness to what it was soon after the second dose was given.
The researchers analyzed 11,000 swab PCR tests performed by the health maintenance organization Maccabi on patients infected with the delta variant. These patients were divided into three groups – people who were never vaccinated, people who became infected within six months of receiving the second dose, and people who became infected after receiving the booster.
“What we found is that the effectiveness of the vaccine in terms of viral load gradually decreases over time, until the end of six months, [viral load] reached a high level, similar to that of an unvaccinated person, ”said Matan Levine-Tiefenbrun, PhD student at Tel Aviv University, also affiliated with Technion and senior researcher. “Nonetheless, we found that the booster brought the viral load back by a factor of four to what it was before.”
The PCR test allows researchers to assess the size of the viral load based on the number of times the DNA sequences of the virus had to be replicated to produce a result. The higher the number of replications required, the lower the initial viral load. Analyzing many of these tests allows researchers to identify broad trends – in this case, the relationship between and how long it has been since the patient’s last dose of vaccine.
Viral load is an important factor in both the likelihood of developing symptomatic illness and the likelihood of transmission, because a person who coughs and sneezes will spread the virus more than an asymptomatic patient would.
The study found that people infected less than two months after their second dose had a lower viral load than unvaccinated people. As a result, they also had milder symptoms and were less contagious.
But after those first two months, the researchers said, immune protection gradually begins to decline and viral loads increase. This process culminates after about six months.
Besides Levine-Tiefenbrun, the other researchers were Professor Roy Kishony and Dr Idan Yellin, both of Techion, and a group of researchers from KSM led by Dr Tal Patalon.
In March, the same group published an article in the journal Nature Medicine showing that Pfizer’s coronavirus vaccine begins to significantly reduce viral load as early as 12 days after the first dose. But that study involved the alpha variant, also known as the British variant, rather than the delta.
“We are finding that the vaccines are also effective in the fourth wave, against the delta variant,” Kishony said. “The effectiveness appears to be very similar to what it was against the UK variant after receiving the first two doses.”
However, he added, the results of the previous study cannot be compared directly to the results of the new study, “because the British variant has been ruled out and gone.”
The new study strengthens data from another Israeli peer-reviewed study that was published last week in the New England Journal of Medicine, and which FDA experts used to discuss the advisability of recommending booster injections in the United States. This study found that the vaccine’s effectiveness in preventing transmission declines dramatically after six months, but even then people who are vaccinated are about 50% less likely to infect others than people who are not vaccinated.
After the booster, however, Pfizer’s vaccine was 95% effective in preventing transmission, according to this study.
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