The risk of contracting Covid after having two doses of the vaccine remains extremely low, according to a new US study.
Two rare cases of Covid-19 from fully-vaccinated people - also known as a "breakthrough infection" - have been reported in the New England Journal of Medicine study on Wednesday.
Both women reportedly had mild symptoms and recovered quickly.
Who were the people who had the so-called 'breakthrough infection' and do they have any medical conditions?
A healthy 51-year-old woman who received her second dose of the Moderna vaccine on February 19 was one of the cases who had a "breakthrough infection".
Nineteen days later, she tested positive for Covid-19 on March 10 after developing symptoms.
The other breakthrough infection was in a healthy 65-year-old woman who received her second dose of the Pfizer vaccine on February 9.
She later learned that her partner, who was unvaccinated, tested positive for Covid-19 on March 3.
In the following days, the woman developed symptoms of her own. She tested positive for Covid-19 on March 17.
Should I be worried about Covid "breakthrough infections"?
No. These "breakthrough infections" of fully-vaccinated people contracting Covid-19 are very rare.
A co-author of the study, Dr Robert Darnell, a senior physician at Rockefeller University in New York City, said there is no huge cause for concern over the two cases.
He said: "They certainly didn't need to be hospitalised. They had at-home cases of Covid-19."
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Have there been further cases of "breakthrough infections"?
The Centers for Disease Control and Prevention (CDC) said last week it had received reports of around 5,800 breakthrough infections out of more that 75 million fully vaccinated people in the US.
The CDC said the "breakthrough infections" occurred in people of all ages who had been vaccinated, but more than 40% of infections were in people 60 or older.
The health agency also said there is some evidence that the full vaccination may make the Covid infection less severe.
Can Covid variants cause "breakthrough infections"?
Scientists have found variants with several differences from the original Covid-19 caused the breakthrough infections.
The South-African variant which has infected one of the patients had the mutation E484K, was first found in the B.1.351 variant.
The E484K has been called an "escape mutant" as it has shown it might be able to escape some of the antibodies produced by coronavirus vaccines.
One of the mutations found in both study participants' infections included D614G, which emerged early in the pandemic.
What have UK scientists said about breakthrough infections?
Professor Graham Medley, from the London School of Hygiene & Tropical Medicine and chairman of Spi-M, has said there will likely "be some breakthrough of immunity", however it is too early to tell as the vaccine has not been around long enough.
He said: "The vaccine hasn’t been around in people in the real world… only in December it started… so we don’t know what effect it’s going to have in three, four months' time and that’s the real unknown.
"So it’s a question of genuine uncertainty."
He added: "The only thing we can be sure of is that we don’t know exactly what is going to happen but we do know that because the vaccine isn’t 100% effective, there will be some transmission, and there will be some breakthrough of immunity."