Vaccination with two doses of the Pfizer jab stays highly effective against severe Covid -- including the Delta variant -- for at least six months, an analysis of US patients said Monday.

While previous data from clinical trials has shown jabs protect against hospitalisation, the study published in the Lancet measures one vaccine's effectiveness over time in a real-world setting. 

Pfizer and healthcare provider Kaiser Permanente looked at records from 3.4 million residents of southern California, about a third of whom were fully vaccinated between December 2020 and August 2021.

After an average period of three to four months, fully vaccinated people were found to be 73 percent protected against infection and 90 percent protected against hospitalisation.

But while protection against infection from Delta fell by 40 percent over five months, protection against hospitalisation involving cases from all variants remained very high for the duration of the study.

The results, the study notes, are consistent with preliminary data from US and Israeli health authorities.

Reduced infection defence is "likely to be primarily due to waning vaccine effectiveness rather than the delta variant escaping vaccine protection", the authors conclude. 

"Our findings underscore the importance of monitoring vaccine effectiveness over time and suggest that booster doses are likely to be needed to restore the initial high amounts of protection observed early in the vaccination programme," it says.

In August the US authorised an extra dose of Covid-19 vaccine for people with weakened immune systems, while in France an extra shot has been offered to the elderly.

Israel has gone further, offering children 12 and older a third dose five months after receiving a second jab.

A September report, however, from the World Health Organization (WHO) found current vaccines are effective enough against severe Covid to make a third jab unnecessary for the general population.

The WHO last month called for a moratorium on booster jabs until the end of the year to address the drastic inequity in dose distribution between rich and poor nations.