Israeli researchers have found that having just one shot of the Pfizer coronavirus vaccine may lead to lower viral loads, making it harder to transmit COVID-19 if someone becomes infected after the first dose.

Key points:

  • Israel’s high vaccination rate makes it easy to collate data
  • Pfizer has an agreement with Israel for anonymised data on vaccine recipients
  • While positive, the studies have not been published or peer reviewed

And it’s not the only positive research about the Pfizer jab to come out of Israel recently.

A separate independent Israeli study, from the country’s largest healthcare provider Clalit, found a 94 per cent drop in symptomatic COVID-19 infections among 600,000 people who received two doses of the Pfizer vaccine.

Researchers also found the fully inoculated group was 92 per cent less likely to develop severe illness from the virus.

Pfizer has said its jab, which will be rolled out to priority groups across Australia from Monday, needs two doses taken 21 days apart to be effective.

The rollout of the Pfizer COVID-19 vaccine will start in Australia on Monday.(AP: Mike Morones)

Why are we getting so much Israeli data?

Nigel McMillan, professor of infectious diseases and immunology at Griffith University’s Menzies Health Institute, said it wasn’t surprising there was an influx of information about the Pfizer jab to come out of Israel.

The Pfizer option was the first coronavirus vaccine to make it through phase three of testing, Professor McMillan explained, which meant it was out being used in the community.

And Israel has already administered more than 6.7 million doses, according to Bloomberg’s COVID vaccine tracker.

This high vaccination rate and the fact that every citizen has a digital health record made it easy for the country to collate and compare information.

“Because [Israel] is vaccinating lots of people, it allows them to compare non-vaccinated and vaccinated people,” Professor McMillan said.

Pfizer has signed an agreement with the Israeli Ministry of Health for anonymised data on vaccine recipients — an arrangement which the company describes as a “non-interventional ‘real-world’ evidence data collection collaboration”, rather than a clinical research study.

Decreased viral loads after one vaccine

The first study, which found reduced viral loads after the first Pfizer dose, retrospectively examined the test results of 2,897 patients.

“What this shows is, if you’re vaccinated, even with just one dose, and immunity isn’t really expected to kick in until at least seven to 10 days … you have less virus in your nasal swabs,” Professor McMillan said.

“So you have about four times less virus,” he said, adding this meant it was less likely the infected person would the transmit the virus.

Peter Collignon, an infectious diseases and microbiology professor at the Australian National University, said the results were not altogether surprising.

“Every vaccine at least decreases the severity of the disease and therefor the amount of virus you probably shed,” he said.

“The Pfizer vaccine’s [impact on disease severity] so far looks the most promising, so I’m not surprised by evidence that it makes an impact on transmission.”

But the data has its limitations. It’s yet to be published or peer reviewed, and does not include data for what happens after the second dose of Pfizer.

“It’s certainly an encouraging trend; if one dose will reduce it four-fold, two doses would be expected to reduce that even more,” Professor McMillan said.

And while the initial findings have been welcomed by Pfizer, the company made clear that two doses of the vaccine were required to provide the 95 per cent efficacy rate observed in its phase three trial, and further research was needed to better understand transmission.

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How will the coronavirus vaccine work?

Drop in symptomatic COVID-19 infections

Meanwhile, the Clalit study found a 94 per cent drop in symptomatic COVID-19 infections among 600,000 people who received two doses of the Pfizer vaccine.

Professor McMillan said although he had not seen the study’s dataset, it would be a positive step if the results were replicated in all vaccine recipients.

The data behind this second study has yet to be publicly released.

Even Pfizer says it’s yet to see the published research from Clalit, but it “looks forward to those results”.

Professor McMillan said during the pandemic, it had been common for research to be released in its early stages because health experts and the broader community were keen to examine any new findings.

However, he maintained that the “gold standard” of scientific research was data that had been peer reviewed and published.