A mild side effect of the Covid booster jabs is the feeling of dehydration, it has been reported.
But other experts say the third vaccine dose has seen less people report fevers, fatigue and aching bodies than after the first two.
Some have said the latest shot leaves them thirsty - while some doctors say that could simply be a symptom of anxiety.
While the side effect has not been confirmed or seen in any clinical trials, one doctor says medics have had a number of reports.
Today marks the first anniversary of the first vaccine given in the UK.
Boosters are currently being given as the key defence against the concerning new Omicron variant.
Dr LaTasha Perkins, a family physician in Washington, says extra thirst is nothing to worry about.
“People have different symptoms when they get their immunisations, but I have heard of people feeling a little bit thirsty," she told Huffington Post :
“That’s not an uncommon finding.”
But Natasha Bhuyan, a physician at primary care US-based practice One Medical, believes there could be other reasons behind the reported dehydration.
“If someone is nervous about the vaccine, that can cause dry mouth, as anxiety is linked to dry mouth.” she said.
It comes as the Centers for Disease Control and Prevention say your third booster shot should be a much smoother experience.
“People had fewer reactions after their third dose than after their second dose,” CDC director Dr. Rochelle Walensky said.
All eligible English adults will be offered a third coronavirus vaccine by the end of January, Boris Johnson promised.
The Prime Minister says military personnel and temporary vaccination centers will be enlisted to help meet the target.
"We’re going to be throwing everything at it in order to ensure that everyone eligible is offered that booster, as I say, in just over two months," Johnson told a Downing Street press conference last week.
National Health Service chief executive Amanda Pritchard said health care staff were working at "breakneck speed" to speed up the rollout.
