Two types of the Omicron known as BA.4 and BA.5 have been labelled as variants of concern in the UK after new evidence on their growth, officials have revealed. The wave of Omicron that hit the UK over the winter involved a form of the virus known as BA.1, with the sub-lineage BA.2 driving a subsequent wave in spring 2022. While the latest data from the Office for National Statistics, released on Thursday, reveals that BA.2 remains dominant in the UK, the UK Health Security Agency (UKHSA) has revealed that two further Omicron sub-lineages, BA.4 and BA.5, have been added to the list of “variants of concern”. BA.4 and BA.5 are currently fuelling a new wave of Covid in South Africa. Dr Meera Chand, UKHSA director of clinical and emerging infections, said the reclassification reflects emerging evidence on the growth of BA.4 and BA.5 internationally and in the UK. “Whilst the impact of these variants is uncertain, the variant classification system aims to identify potential risk as early as possible,” she said. “UKHSA is undertaking further detailed studies. Data and analysis will be released in due course through our regular surveillance reporting.” According to the UKHSA, as of Thursday there had been 115 confirmed cases of BA.4 and 80 cases of BA.5 in the UK. However, a report by the agency reveals modelling suggests they have a growth advantage over BA.2, including in the UK. In other words they could end up outcompeting the current dominant form of the coronavirus. While the report cautions there is a higher degree of uncertainty, a preprint from a team including Prof Tulio de Oliveira, director of the Centre for Epidemic Response and Innovation at Stellenbosch University, suggested BA.4 and BA.5 appear to have a similar growth advantage over BA.2 as it had over BA.1. The UKHSA noted that while there can be several reasons for a growth advantage, it appears a degree of immune escape is likely contributing in the case of BA.4 and BA.5. Research has previously shown the BA.4 and BA.5 have the same mutations in their spike protein – the part of the virus that helps it to enter cells. Among their mutations are those that may help them dodge the body’s immune responses, including one found in the Delta variant called L452R. The UKHSA said that, at present, there is no data to determine the impact of BA.4 and BA.5 on the hospital admissions in the UK. However, experts in South Africa have previously suggested there is little indication these new sub-lineages will be more severe than other Omicron sub-lineages, a view that has been echoed by the European Centre for Disease Prevention and Control (ECDC). “Based on the limited data currently available, no significant increase in infection severity compared to the circulating lineages BA.1 and BA.2 is expected,” the ECDC stated last week. “However, as in previous waves, if Covid-19 case numbers increase substantially, some level of increased hospital and ICU admissions is likely to follow.”