Pfizer says third vaccine dose likely boosts antibodies against Delta variant

A third dose of the Covid-19 vaccine developed by Pfizer and BioNTech will likely significantly boost antibody levels in patients against the so-called Delta variant of the virus.

After the close of US markets on Thursday, the two companies said initial data from a booster trial showed that a third dose of BNT 162b2 was safe and increased neutralising antibodies five to 10-fold against wild-type Covid-19 and the Beta variant, the latter of which was first discovered in South Africa.

Furthermore, lab tests had shown that their jab produced a "strong" neutralising response against the Delta variant, which was first detected in India, and they expected that a third dose would improve protection against it by a similar amount as seen against Beta.

Pfizer's head of research, Mikael Dolsten, doubled down on that assertion, saying in an interview with Bloomberg that: "there is a lot of concern [about variants]. We are confident that such a boost will be highly effective against the delta variant."

The two companies also said they were developing an updated version of the vaccine that targets the full spike protein of the Delta variant.

Clinical studies were set to begin in August - subject to receiving the necessary regulatory approvals.

Pointing to recent data from Israel's Ministry of Health, Pfizer and BioNTech said the decline in vaccine efficacy in preventing infection and symptomatic disease after six months meant that it was likely that a third vaccine dose might be needed six to 12 months after full vaccination.

A study in the UK in May had shown that BNT 162b2 was 88% effective against symptomatic disease from the Delta variant, whereas the Israeli study from earlier in the week revealed a drop in efficacy to 64%.

Dolsten said Pfizer's interpretation of the Israeli data was that blood-antibody levels had decreased since people were vaccinated in January and February.

"When you have low blood levels of an antibody, viruses that are highly contagious may reinfect and cause mild disease," he reportedly said.

People did however retain protection against severe disease.

Indeed, he believed it unlikely that a customised jab against Delta would be needed.

Nonetheless, Dolsten added: "For the next few years, it is better to be prepared for what seems like a sober reality than to hold for miracles."