A California nurse has been diagnosed with COVID more than a week after being administered Pfizer's coronavirus vaccine, The Independent and other news organizations reported Thursday.

However, scientists say the human body needs more time to defend against the disease.

Based on 10 News San Diego report, the 45-year-old nurse who works at two different hospitals, felt ill around a week after getting his Pfizer shot.

He began experiencing muscle aches, chills and fatigue Christmas Eve and sought a COVID test on Christmas. The result came back positive.

Health authorities said different dynamics might have played into the result despite the nurse being immunized against the virus.

Experts say the incubation period of the vaccine could take as long as two weeks for the patient to start to build up protection from COVID.

Dr. Christian Ramers, an infectious disease specialist with The Family Health Centers of San Diego, said that the nurse's condition is not unexpected. "This is exactly what we would expect to happen if someone was exposed," he said.

Ramers said the first dose of the vaccine will give around "50%" protection, while the second dose will increase that to around "95%."

Pfizer revealed in November that the vaccine it was developing with German pharmaceutical BioNTech was 95% effective against the disease and it hoped to get emergency use authorization to curb the spread of the virus.

"This is an extraordinarily strong protection," Dr. Ugur Sahin, BioNTech's chief executive officer and co-founder, said.

As of Thursday, the total number of COVID cases worldwide is 82.5 million, with 46.6 million recoveries and 1.8 million deaths.