At least one of the studies also demonstrated that being fully vaccinated matters.
The first nationwide study of coronavirus vaccination, done in Israel, showed Pfizer/BioNtech's vaccine works far better after two doses.
Two shots of the vaccine provided greater than 95% protection from infection, severe illness and death, Dr. Eric Haas of the Israel Ministry of Health and colleagues reported in the Lancet medical journal.
"Two doses of BNT162b2 are highly effective across all age groups in preventing symptomatic and asymptomatic SARS-CoV-2 infections and COVID-19-related hospitalizations, severe disease, and death, including those caused by the B.1.1.7 SARS-CoV-2 variant," they wrote.
The B.1.1.7 variant, first seen in Britain, has spread widely and is now the most common new variant seen in the US. It was also common in Israel when the study was done. The B.1.351 variant, first seen in South Africa, was also circulating in Israel, but had not caused many infections at the time of the study, so the researchers were unable to say how well the vaccine worked against that variant in particular.
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With its small population, Israel was able to quickly vaccinate much of its population.
The study found the Pfizer vaccine provided 95.3% protection against infection and 96.7% protection against death seven days after the second dose.
"By 14 days after vaccination, protections conferred by a second dose increased to 96.5% protection against infection, 98% against hospitalization, and 98.1% against death," the team wrote.
But people who got only one dose of the vaccine were far less protected. One dose alone gave just 57.7% protection against infection, 75.7% against hospitalization, and 77% against death.
"Topping up your immunity with the vaccine boost will be even more important with the emergence of new variants that might have acquired genetic changes that make them more resistant to the immunity generated by vaccines or following natural infection," said Jonathan Ball, a professor of molecular virology at Britain's University of Nottingham, who was not involved in the research.
Separately, a team in the Gulf state of Qatar looked at the efficacy of Pfizer's vaccine in the population there when B.1.351 and B.1.1.7 were both circulating.
They found reassuring results.
"The estimated effectiveness of the vaccine against any documented infection with the B.1.1.7 variant was 89.5% at 14 or more days after the second dose. The effectiveness against any documented infection with the B.1.351 variant was 75%," the researchers wrote in a letter to the New England Journal of Medicine.
Many of the new variants seen around the world have developed mutations that help them at least partly evade the human immune system, including the immune response elicited by vaccines. Health officials are worried the virus could change even more into forms that more strongly resist the effects of vaccines -- much as influenza does most years.
So manufacturers are already making and testing formulations against the most worrying of the variants. The design of the new mRNA vaccines made by Moderna and Pfizer make this easier than it would have been in the past. The genetic material used as the basis of the vaccines is made in a lab and the sequence is easily tweaked.
Vaccine maker Moderna reported Wednesday that a booster shot delivering a half-dose of its current vaccine revs up the immune response against both B.1.351 and P.1.
And a booster dose formulated specifically to match B.1.351 was even more effective, Moderna said in a statement.
Moderna tested booster doses of either its current vaccine or a version designed specifically against B.1.351 in a Phase 2 trial of 40 people who had already been vaccinated six to eight months before. Blood tests showed half these volunteers had a low antibody response against B.1.351 and the P.1 variant before they got the booster shot.
Two weeks after the booster, their antibody levels had grown against the so-called wild type coronavirus -- the variant most common around the world -- as well as B.1.351 and P.1, Moderna said in the statement.
"The majority of adverse events were mild or moderate in severity," the company said.
"We are encouraged by these new data, which reinforce our confidence that our booster strategy should be protective against these newly detected variants. The strong and rapid boost in titers to levels above primary vaccination also clearly demonstrates the ability of mRNA-1273 to induce immune memory," Moderna CEO Stéphane Bancel said in the statement.
The company is also testing a vaccine booster that combines the original formulation with the B.1.351 specific formula.
"We will continue to make as many updates to our COVID-19 vaccine as necessary to control the pandemic," Bancel said.
The company has submitted the data, collected as part of a Phase 2 trial, to a pre-print site and said it would submit for publication in a peer-reviewed journal when it gets more data from the trial.
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In another study, vaccine maker Novavax confirmed earlier findings that showed its vaccine protects against B.1.351.
The study, published in the New England Journal of Medicine, confirmed what the company has announced previously -- that in South Africa, Novavax's vaccine has an efficacy of about 49% among adults in general, regardless of their HIV status, and of 60% among HIV-negative adults.
The company released updated findings from the trial in March in which it found the vaccine lowered the risk of symptomatic infection by 55% among healthy adults without HIV -- which suppresses the immune system. That updated data is not included in Wednesday's publication.
Novavax has not yet asked the US Food and Drug Administration for emergency use authorization of its vaccine, which is made using a different formulation than vaccines made by Moderna, Pfizer/BioNTech and Johnson & Johnson.