THE emergence of the Omicron Covid variant at the end of last year marked a major turning point in the pandemic.

On the one hand, it ushered out the more virulent but less contagious Delta strain which meant that a much smaller proportion of infections resulted in serious illness.

Less favourably, however, its sheer transmissibility meant that it was able to spread faster and infect far more people.

This "growth advantage" appears to be rooted in mutations to the spike protein, which have continued to proliferate as more and more Omicron offshoots have evolved.

Just as BA.1 Omicron had outcompeted Delta, BA.2 replaced BA.1, and then BA.2 was pushed aside by BA.5 and (to a lesser extent) BA.4.

With each new generation the selective advantage has been immune evasion - mutations to the spike protein which make Omicron more and more unrecognisable to Covid antibodies.

READ MORE: Cases of fast-spreading BA.2.75 'Centaurus' found in Scotland 

This is especially the case for antibodies induced by vaccines based on the original Wuhan strain, but even prior infection with Omicron BA.1 appears to offer little protection against reinfection with one of its newer sublineages.

This has led to scientists describing Omicron as a "particularly stealthy immune invader", because it leaves little imprint on the immune memory.

The effect has been successive, large waves of infections and reinfections just two or three months apart which pile pressure on the NHS through staff absences and bed losses. Luckily, cellular immunity (from T cells) continues to provide strong protection against serious illness.

The BA.5 wave now appears to be receding in Scotland, but many expect another Omicron wave around September - potentially accompanied by a resurgence in flu.

Could it be led by BA.2.75? The BA.5 variant was first detected in Scotland in March, and became dominant in June, so the timescale could fit.

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The picture from India is hazy, however: it is unclear whether 'Centaurus' genuinely is outcompeting other forms of Omicron or only spreading rapidly in regions where it is not up against BA.5.

There is also some evidence that the recent BA.2.75 spike is already levelling off - although others caution that testing levels are too low now to accurately gauge its transmissibility.

Of course, Covid may throw us another curveball. This time last year, many experts believed Delta represented an evolutionary peak for the virus, only for Omicron to move the goalposts yet again.