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 has found.

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% protected against infection and 90% protected against hospitalisation.

But while protection against infection from Delta fell by 40% 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."

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.

The WHO's technical lead for Covid-19 has also warned that "we're not out of the woods" in the fight against the pandemic, even if many people thought it was over.

"The situation is still incredibly dynamic", Maria Van Kerkhove said during a live presentation on the WHO's social media channels.

"It's dynamic because we don't have control over this virus. We're not out of the woods yet, we're very much in the middle of this pandemic.

"But where in the middle... we're not quite sure yet because frankly, we're not using the tools we have right now to get us closer to the end."

She added: "What I really struggle with is in some cities we see ICUs and hospitals full and people dying - yet on the streets people are acting like it's completely over.

"You can't have it both ways."

J&J files for authorisation of vaccine booster

Meanwhile, Johnson & Johnson has said it had submitted data to the US Food and Drug Administration for emergency use authorisation of a booster shot of its vaccine in people aged 18 years and older.

The filing comes after the FDA last week scheduled a meeting of its expert advisory committee for 15 October to discuss whether to authorise a second shot of J&J's single-dose vaccine.

J&J said its submission includes data from a late-stage study that found a booster of its vaccine given 56 days after the primary dose provided 94% protection against symptomatic Covid-19 in the US and 100% protection against severe disease, at least 14 days after the booster shot.

Moderna also submitted its application seeking authorisation for a booster shot of its two-dose vaccine last month.

J&J said it plans to submit the data to other regulators, the WHO and National Immunisation Technical Advisory Groups to inform decision-making on local vaccine administration strategies, as needed.