Covid-19 doesn't follow normal seasonal patterns, like other respiratory viruses – waves of infection can happen at any time of year.

Every July for the past four years, epidemiologists at the US Centers for Disease Control and Prevention (CDC) have noted a sudden spike in Covid-19 cases and hospitalisations, an annual trend which has been dubbed "the summertime surge".

This summer in the United States, Covid-19 rates are reportedly particularly high in Arizona, California, Hawaii and Nevada. In these western states, the number of positive tests reached 15.6% on the week ending July 6th, 1% up on the previous week. The CDC’s investigations show that viral rates in wastewater are also on the rise once again.

A similar trend has played out on the other side of the Atlantic, where according to the UK's Health Security Agency, positive Covid-19 tests rose from 4% at the end of March to 14% by the end of June.

The recent rise has been attributed to the FLiRT subvariants, the latest evolution of the Omicron strain of Covid-19 which emerged towards the end of 2021. This new raft of subvariants is the inevitable consequence of the virus trying out new mutations in its spike protein – a structure which is essential for Covid-19's ability to enter human cells, but can also alert the human immune system to the virus' presence in the body. As a result, Covid-19 has found more effective ways of evading the neutralising antibodies within most of our immune systems, while still being capable of locking onto the ACE2 receptor, a protein on human cells in the respiratory tract which allows it to enter the body. (Find out more about how Covid-19's symptoms have changed as new varients have emerged.)

According to Shan-Lu Liu, who directs the Viruses and Emerging Pathogens Program at The Ohio State University and has studied the FLiRT subvariants, these latest Covid-19 viruses have managed to strike a balance between escaping the immune system and still being able to bind to cells, which is driving many of the new cases.

“The elderly and immunosuppressed are particularly vulnerable to new subvariants due to their weakened immune responses to vaccination and natural infection," says Liu. He explains that experts recommend that these groups receive booster shots including the XBB.1.5 monovalent vaccine. This was designed to target the Omicron XBB 1.5 subvariant of Covid-19, which emerged in 2022.

Given the need to protect society’s most vulnerable, studying these continually emerging variants remains crucial for updating the world’s Covid-19 vaccines. Based on work done by Liu and others, US regulators and the World Health Organization have been able to make specific recommendations regarding new vaccine targets based on the latest spike protein mutations, in time for the latest Covid vaccine rollout in the autumn.

Yet for the scientists who monitor how SARS-CoV-2 is evolving and changing, it is still almost impossible to predict when the next strains of note will emerge. While most common respiratory infections like........

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