COVID Booster Shot Reduces Viral Load, Limits Transmission, Israeli Study Finds

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A vaccination center in Jerusalem, last month
A vaccination center in Jerusalem, last monthCredit: Emil Salman

A booster shot of Pfizer’s coronavirus vaccine significantly reduces viral load in patients infected with the delta variant, and therefore reduces the chances of transmission, a new Israeli study has found.

The study was conducted 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 papers that haven’t 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 at reducing viral load dissipates. But a third dose slashes viral loads by a factor of four, thereby restoring the vaccine’s effectiveness to what it was shortly after the second dose was administered.

The researchers analyzed 11,000 PCR swab tests conducted by the Maccabi health maintenance organization on patients who had been infected with the delta variant. These patients were divided into three groups – people who were never vaccinated, people who were infected within six months of getting the second dose and people who were infected after getting the booster shot.

“What we discovered is that the vaccine’s effectiveness with respect to viral load gradually wanes over time, until after six months, [viral load] reaches a high level, similar to that of an unvaccinated person,” said Matan Levine-Tiefenbrun, a doctoral student at Tel Aviv University who is also affiliated with the Technion and was the lead researcher. “Nevertheless, we discovered that the booster shot brings the viral load back down by a factor of four, to what it was before.”

A medical worker prepares a coronavirus vaccine dose in Jerusalem, last month.Credit: Ohad Zwigenberg

The PCR test enables researchers to assess the size of the viral load based on how many times sequences of the virus’ DNA needed to be replicated to produce a result. The greater the number of replications required, the lower the initial viral load was. Analyzing large numbers of such tests enables researchers to identify broad trends – in this case, the relationship between and how long it has been since the patient’s last vaccine dose.

Viral load is a significant factor in both the likelihood of developing symptomatic illness and the likelihood of transmission, since someone who is coughing and sneezing 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 lower viral loads than unvaccinated people. Consequently, they also had milder symptoms and were less infectious.

But after those first two months, the researchers said, immune protection gradually begins waning and viral loads rise. This process peaks after about six months.

Aside from Levine-Tiefenbrun, the other researchers were Prof. Roy Kishony and Dr. Idan Yellin, both of the Techion, and a group of researchers from KSM led by Dr. Tal Patalon.

In March, this same group published an article in the journal Nature Medicine showing that Pfizer’s coronavirus vaccine starts significantly reducing viral load as early as 12 days after the first dose. But that study involved the alpha variant, also known as the U.K. variant, rather than delta.

“We’re seeing that the vaccines are also effective in the fourth wave, against the delta variant,” Kishony said. “The effectiveness seems very similar to what it was against the British variant after receipt of the first two doses.”

However, he added, the results of the earlier study can’t be compared directly to the results of the new study, “because the British variant has been pushed aside and disappeared.”

The new study bolsters the data from another Israeli study, this one peer-reviewed, that was published last week in the New England Journal of Medicine, and which FDA experts made use of in discussing whether to recommend booster shots in the United States. That study found that the vaccine’s effectiveness in preventing transmission declines significantly after six months, but even then, vaccinated people are roughly 50 percent less likely to infect others than unvaccinated people.

After the booster, however, Pfizer’s vaccine is 95 percent effective in preventing transmission, that study said.

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