Why Covid variants BA.4 and BA.5 are driving the UK’s new Omicron wave

The Omicron variant BA.5 has become dominant in many parts of the country, outpacing even its highly contagious sibling BA.4 - but why is it spreading so fast and what could the impact be on the NHS?

Covid is back. A minister has warned restrictions could return, after the number of people infected reached 2.7 million across the UK, according to the latest weekly estimates.

The current wave is being driven by the Omicron variants BA.4 and BA.5, but how are they different from previous variants, and from each other?

Here is everything you need to know.

Omicron sub-variants BA.4 and BA.5 are currently dominant in the UK

Covid is always mutating and while some new variants are weaker and die out quickly, others take hold and cause a fresh wave of infections. This is often because they have an advantage over previous strains, either because they are more contagious or they evade the immune response built up in our bodies by previous infection or vaccines.

In the UK, the main waves were caused first by the original Wuhan strain, then by Alpha, Delta, Omicron, Omicron BA.2 (Stealth Omicron) and now Omicron BA.4 and BA.5.

BA.4 and BA.5, taken together, now make up the vast majority of Covid cases across all four nations of the UK, according to the latest mass testing by the Office for National Statistics (ONS).


This also reveals working-age adults are the most likely to be infected with Covid-19, but that infections are rising in all age bands.

BA.5 is expected to win out over BA.4

Omicron BA.5 has a slight growth advantage over BA.4, according to health officials at the UK Health Security Agency (UKHSA).

This means it is “most likely that BA.5 will become the dominant variant in the UK”, their latest report into Covid variants says.

BA.5 has already become the dominant strain in many parts of England, research shows.

Each week, the Wellcome Sanger Institute analyses Covid samples from all parts of England and feeds the data back to the UKHSA. Its latest findings, for the week ending June 25, shows BA.5 made up the majority of analysed Covid samples in the East Midlands, East of England, London, South East, South West, West Midlands and Yorkshire and the Humber.


The highest proportion of BA.5 was found in Yorkshire and the Humber, at 61%.

Previous infection offers less protection

BA.4 and BA.5 were designated as variants of concern on 18 May. They are both subvariants of Omicron and are very similar to each other, meaning they are often talked about together.

Before their rise, the most recent dominant strain was BA.2, at first dubbed ‘Stealth Omicron’ because it was harder to tell apart from Delta.

Both BA.4 and BA.5 both have a ‘growth advantage’ over BA.2, meaning they spread more easily.

And there is evidence that previous Covid infections do not protect people particularly well against catching the new variant. Meanwhile, antibodies from being triple-vaccinated show a “similar or lower” chance of fighting off BA.4, compared with the first Omicron strain, the UKHSA says. Vaccination is still thought to protect against severe disease.

There is currently no evidence that Omicron BA.4 and BA.5 cause more severe illness than previous variants.

Infectiousness ‘similar to measles’

BA.4 and BA.5 are far more contagious than previous coronavirus variants, according to scientists.

Writing for the The Conversation, Adrian Esterman, an epidemiologist and professor of biostatistics and epidemiologist at the University of South Australia, said: “We measure how contagious a disease is by the basic reproduction number (R0). This is the average number of people an initial case infects in a population with no immunity (from vaccines or previous infection).

“New mutations give the virus an advantage if they can increase transmissibility: the original Wuhan strain has an R0 of 3.3, Delta has an R0 of 5.1, Omicron BA.1 has an R0 of 9.5, BA.2, which is the dominant subvariant in Australia at the moment, is 1.4 times more transmissible than BA.1, and so has an R0 of about 13.3.

“A pre-print publication from South Africa suggests BA.4/5 has a growth advantage over BA.2 similar to the growth advantage of BA.2 over BA.1. That would give it an R0 of 18.6.


“This is similar to measles, which was until now our most infectious viral disease.”

Fewer deaths than previous waves

The number of deaths from coronavirus have not reached the peaks seen in previous waves.


However, the number of Covid patients admitted to hospitals has been on the rise for weeks, placing the NHS under increasing strain.


Professor Susan Hopkins, Chief Medical Advisor at UKHSA said: “It is clear that the increasing prevalence of Omicron BA.4 and BA.5 are significantly increasing the case numbers we have observed in recent weeks. We have seen a rise in hospital admissions in line with community infections but vaccinations are continuing to keep ICU admissions and deaths at low levels.”

She said that vaccinations remain “our best form of defence against the virus”, adding: “Our data also show that 17.5 per cent of people aged 75 years and over have not had a vaccine within the past six months, putting them more at risk of severe disease. We urge these people in particular to get up-to-date.”