Covid: BA.2.86 'one of the most immune evasive' variants

Researchers around the globe have been racing to untangle the significance of the highly-mutated BA.2.86 variant since it was first detected in Denmark in July.

To date, six cases have been confirmed through testing in Scotland and traces have also been picked up in three health board regions through wastewater surveillance.

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Covid levels have been increasing since the beginning of July (Image: Getty)

Testing - both in Scotland and the rest of the UK - has been massively scaled back, however, so it is much harder to gauge the true prevalence of this new strain and whether it is on its way to become the dominant cause of Covid infections.

The most recently available sequencing data for Scotland - covering 266 cases in the week ending August 13 - shows that around 85% cases were either some form of Omicron XBB, which has been dominant for the past six months, or the newer EG.5.1 'Eris' strain.

The first known case of BA.2.86 in Scotland was not detected until August 26, although it had probably been in circulation for weeks by then.

Testing of symptomatic hospital patients shows that the number of people in hospital with Covid (though not necessarily because of Covid) rose steadily from 66 on July 1 to 420 by September 10.

Wastewater surveillance shows Covid levels at their highest for more than a year in August (Image: PHS)

Wastewater levels of the virus also increased sharply during August, reaching a 14-month peak in the middle of the month, before falling back.

There are now signs that the number of people in hospital with Covid may also be in decline - dipping to 367 on September 17 - but it is too early to tell whether this is the start of a downward trend.

It is also unclear how much of a role - if any - the BA.2.86 'Pirola' variant played in the recent increase.

The analysis carried out by scientists in Tokyo's Sato Lab, and published in the journal Lancet Infectious Diseases on Monday, offers the most recent insight into how this new strain might behave.

They estimate that its reproduction number - a measure of how prolifically a virus transmits from an infected person into new hosts - is "comparable to or even greater than that of EG.5.1, one of the most rapidly expanding XBB subvariants".

Back at the beginning of the pandemic, Covid had an R number of around three - meaning that, on average, each infected person would pass the virus on to three others.

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Today, the virus has mutated so much that the R number would exceed 10 under circumstances where the entire population were susceptible (no vaccine protection or immunity from prior infections) and zero mitigations, such as lockdown, were in place.

In reality, the vast majority of people have hybrid immunity from vaccines and, in some cases, multiple prior infections.

Most people in Scotland have been vaccinated against Covid as well as acquiring immunity through infection, meaning the risk of acute illness is much less (Image: PA)

This blunts the ability of Pirola to spread, but it also means that a process of natural selection is driving the virus to continually adapt.

Pirola - an offshoot from the Omicron BA.2 lineage - has gained an extra 34 mutations on its spike protein in a single evolutionary leap, suggesting that antibodies will be less able to bind on to prevent the virus infecting cells.

One hypothesis is that it acquired these mutations during a long-term infection in an immunocompromised individual resident in a part of the world with little or no surveillance - possibly a year or more ago - before spreading into the local community and then being exported, unnoticed, around the world.

It has already been found as far afield as Israel, South Africa, Canada, and Thailand.

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The Sato Lab used BA.2.86 'pseudoviruses' - artificial viruses engineered to express the BA.2.86 spike protein - to test how Pirola might interact with antibodies.

Using antibody-rich blood serum obtained from individuals vaccinated multiple times, up to and including the most recent BA.1 and BA.5 bivalent mRNA vaccines developed by Pfizer and Moderna specifically to target Omicron, the Japanese team detected "very little or no antiviral effects against BA.2.86".

Repeating the test using blood serum from patients who had been treated with the monoclonal antibody therapies bebtelovimab, sotrovimab, and cilgavimab - all of which work against Omicron BA.2 - the team found that it "did not exhibit antiviral effects against BA.2.86".

Forms of Omicron XBB have been the main strains in circulation in Scotland this year (Image: PHS)

Finally, they used blood serum obtained from vaccinated individuals known to have had breakthrough infections caused by an XBB sublineage of Omicron, which has been dominant in Scotland in some form (XBB, XBB.1.5 - nicknamed 'Kraken' - or XBB.1.16) since early 2023.

Echoing other recent laboratory studies, these naturally occurring XBB antibodies showed some ability to neutralise the BA.2.86.

This is reasonably good news for anyone who has had Covid in the past six months as the likelihood is that they will be carrying these XBB antibodies, although this line of defence diminishes over time.

The UK's medicines regulator, the MHRA, has recently approved Pfizer's new XBB-targeted vaccine although it has not yet been incorporated into the rollout.

Growing evidence, including that from the Sato Lab, makes a plausible case for accelerating that ahead of winter.

Antibodies are only one part of the immune system, however.

There is no evidence that T cells - which attack the virus once it has invaded cells - are less effective, which in turn should mean that existing vaccines hold up against severe disease.

Scientists in Tokyo have been examining the BA.2.86 'Pirola' Covid variant (Image: Getty)

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There are still question marks over the virulence of BA.2.86.

A Covid outbreak at a Norfolk care home in August was investigated after 33 out of the 38 residents tested positive, with reportedly more severe symptoms. Sequencing showed that BA.2.86 was behind 22 out of the 33 residents' infections, included one person who was hospitalised.

However, there is no data indicating that hospitals are seeing a spike in people sick with Covid.

Pirola is the most striking variant to emerge since Omicron, but - this far into the pandemic - even that doesn't necessarily make it a gamechanger.