Pfizer's Covid-19 vaccine's overall protection against the disease decreases after six months, according to a study.

Its effectiveness drops to around 84% for immunized people about four to six months after receiving their second dose, Pfizer chief executive officer Albert Bourla said.

The vaccine was 97% effective at preventing severe illness for at least six months but its effectiveness against any symptomatic illness dropped by around 6% every two months, the report said.

The pre-print report of the study released this week suggests a gradual "declining trend in vaccine efficacy" over six months after two doses in more than 45,000 people worldwide, according to WebMD.

"We have also seen data from Israel that there's a waning of immunity and that starts affecting what used to be what was 100% against hospitalization," Bourla said.

"Now, after the six-month period, the effectiveness is becoming low 90s and mid-to-high 80s," Bourla added.

Dr. Stephen Thomas, lead study author, said he was "generally encouraged by the results of the paper."

Thomas is a coordinating investigator for the Pfizer vaccine trial and chief of the SUNY Upstate Institute for Global Health and Translational Science in New York.

Thomas said the anticipation was always that the vaccine's protection was going to wane.

The big question, he said, was whether the vaccine's effectiveness would drop to a degree that would affect the so-called "public health burden of the virus," specifically hospitalizations and deaths.

"So far that doesn't appear to be the case," he said in quotes by NBC News.

The good news, according Bourla, is the company is "very, very confident  a third dose, a booster, will be enough to protect against the Delta variant.

Meanwhile, the Centers for Disease Control and the World Health Organization do not recommend booster shots at this time.