An expert is warning COVID-19 vaccinations don't provide instant protection after a US nurse tested positive just days after receiving his first dose.
Californian ER nurse Matthew W told ABC 10 News he had received the first of two required Pfizer coronavirus vaccines the week before Christmas.
He said his arm was sore for a day but he suffered no other side effects and continued to work in a COVID-19 patient unit.
Then on Christmas Eve (US time), he became sick, with a fever as well as muscle aches and fatigue.
On Boxing Day he got tested for COVID-19 and it came back positive.
Dr Christian Ramers, an infectious disease specialist with Family Health Centers of San Diego, told ABC he wasn't surprised to hear of the infection.
"It's not unexpected at all. If you work through the numbers, this is exactly what we’d expect to happen if someone was exposed," he said.
"That first dose we think gives you somewhere around 50 percent, and you need that second dose to get up to 95 percent."
Dr Ramers said he knows of several other cases where health care workers became infected around the time they received the vaccine, ABC reported.
He said it shows it the results of the vaccine aren't immediate.
"We know from the vaccine clinical trials that it’s going to take about 10 to 14 days for you to start to develop protection from the vaccine."
The United States have so far recorded more than 20,216,000 cases of COVID-19 and 350,000 deaths.