By the start of April, nearly 5 million people in Israel had received two doses of the Pfizer-BioNTech jab, more than 70 per cent of the population.

The study found that two doses conveyed 95.3 per cent protection against infection and 96.7 per cent protection against death seven days after the second dose.

After 14 days, that protection increased to 96.5 per cent and 98 per cent, respectively.

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SCMP Explains: What’s in a Covid-19 vaccine?

SCMP Explains: What’s in a Covid-19 vaccine?

But the protection was considerably lower when people received just a single vaccine dose.

Between seven and 14 days after the first dose, protection against infection was found to be 57.7 per cent, and protection against death 77 per cent.

The authors said that one dose may provide a shorter window of protection, especially in an environment where new viral variants emerge.

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“Importantly, the study shows that two doses of the vaccine significantly increase levels of immunity and protection,” said Jonathan Ball, professor of Molecular Virology at the University of Nottingham, who was not involved in the research.

“This is why it is important that people get both doses.”

During the analysis period, there were 232,268 confirmed Covid-19 infections, and nearly 95 per cent of samples tested were found to be the English B117 variant. There were 4,481 severe infections and 1,113 deaths.

The team behind the research said that they were not able to study the effect of the South African variant, which has also been identified in Israel.

Before Thursday, the largest real-world study of the Pfizer-BioNTech vaccine, also in Israel, was nearly five times smaller, involving 1.2 million people.

Writing in a comment article, Eyal Lesham of the Chaim Sheba Medical Centre and Annelies Wilder-Smith of the London School of Hygiene and Tropical Medicine, said the study findings “suggest that high vaccine coverage rates could offer a way out of the pandemic”.

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“Regrettably, rapid population-level coverage cannot be easily replicated in many other countries,” said the pair, who were not involved in The Lancet study.

“The global use of (Pfizer-BioNTech) vaccine is limited by supply issues, high costs, and ultra-cold chain storage requirements.”

Meanwhile, US biotech firm Moderna said that early human trial data showed that a third dose of either its current Covid-19 shot or an experimental new vaccine candidate increases immunity against variants of Covid-19 first found in Brazil and South Africa.

The booster shots, given to volunteers previously inoculated with Moderna’s two-dose vaccine regimen, also boosted antibodies against the original version of Covid-19, Moderna said.

The early data comes from a 40-person trial testing both Moderna’s existing shot and a version developed to protect against a South African variant of Covid-19. Moderna is also studying a shot that combines both the new and existing vaccine.

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The results show that while booster shots of either version of the vaccine increased antibodies against all of the variants of Covid-19 tested in the trial, the new booster had a bigger response against the South African variant than the original vaccine.

“We are encouraged by these new data, which reinforce our confidence that our booster strategy should be protective” against the newer variants of Covid-19, Stephane Bancel, Moderna’s chief executive officer, said in a statement.

Both booster shots were well tolerated, with side effects similar to what volunteers in previous studies experienced from the second dose of its vaccine, Moderna said.

The new variants of Covid-19 first discovered in South Africa and Brazil are thought to be more resistant to existing vaccines.

Additional reporting by Reuters