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How jabs work against Delta Covid variant

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Vaccines retain important effectiveness against the Delta variant after two doses. AFP

How jabs work against Delta Covid variant

The Delta variant of the coronavirus, first identified in India, is a cause for global concern with studies showing it is more contagious and resistant to vaccines than other forms of the virus.

But there is also evidence that vaccines retain important effectiveness against Delta after two doses.

A British study published in the Lancet medical journal early this month looked at levels of neutralising antibodies produced in vaccinated people exposed to the Delta, Alpha (first identified in Britain) and Beta (first identified in South Africa) variants.

It found that antibody levels in people with two doses of the Pfizer/BioNTech shot were six times lower in the presence of the Delta variant than in the presence of the original coronavirus strain on which the vaccine was based.

The Alpha and Beta variants also provoked lower responses, with 2.6 times fewer antibodies for Alpha and 4.9 times fewer for Beta.

A French study from the Pasteur Institute concluded that neutralising antibodies produced by vaccination with the Pfizer/BioNTech jab are three to six times less effective against the Delta variant than against the Alpha variant.

Although they represent an essential marker, the levels of antibodies measured in a lab are not enough to determine the efficacy of a vaccine.

In particular they do not take into account a second immune response in the form of killer T cells – which attack already-infected cells and not the virus itself.

As a result, real-world observations are crucial to measuring vaccine effectiveness – and the first results are reassuring.

According to data published on June 14 by Public Health England, vaccination with Pfizer/BioNTech and AstraZeneca is as effective at preventing hospitalisation in the case of the Delta variant as it is in the case of the Alpha variant.

Two doses of the Pfizer/BioNTech jab prevent 96 per cent of hospitalisations due to the Delta variant, while AstraZeneca prevents 92 per cent, according to a study involving 14,000 people.

Previous data released by British health authorities at the end of last month come to similar conclusions for less serious forms of the illness.

The Pfizer/BioNTech vaccine is 88 per cent effective against symptomatic Covid caused by the Delta variant two weeks after the second dose, while the jab is 93 per cent effective for cases caused by the Alpha variant.

AstraZeneca shows efficacy of 60 per cent against cases caused by the Delta variant and 66 per cent in the case of Alpha.

Scottish authorities published similar data on June 14 in the Lancet.

The team behind the Sputnik V jab meanwhile tweeted on June 15 that theirs was “more efficient against the Delta variant … than any other vaccine that published results on this strain so far”.

They did not publish results but said the study by the Gamaleya Center, a Russian research institute, had been submitted for publication in an international peer-reviewed journal.

Among authorised vaccines only one – developed by Janssen – is given in one dose instead of two, and not enough data exists to determine its effectiveness against the Delta variant.

For the others, lab and real-world tests both conclude that one dose of any vaccine only gives limited protection against the Delta variant.

“After a single dose of Pfizer-BioNTech, 79 per cent of people had a quantifiable neutralising antibody response against the original strain, but this fell to … 32 per cent for B.1.617.2 [Delta],” says the lab study from this month.

The Pasteur Institute found that a single dose of AstraZeneca would have “little to no efficacy” against the Delta variant.

Data from the British government confirms the tendency in real-world scenarios – both vaccines were 33 per cent effective against symptomatic cases caused by Delta three weeks after the first dose compared to around 50 per cent effectiveness against the Alpha variant.

In the UK – where the Delta variant is now responsible for 96 per cent of new cases – these findings pushed the government on June 14 to reduce the waiting period between doses from 12 weeks to eight for people over 40.

In France the wait has been reduced to three weeks from five for a second dose of the Pfizer/BioNTech and Moderna vaccines.

The Pfizer/BioNTech jab does however offer very high (94 per cent) protection against hospitalisation due to the Delta variant after one dose.

Scientists agree that the best defence against the Delta variant is to get a full two-dose vaccination against coronavirus.

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