The Omicron variant of Covid-19 sparked a return of stricter regulations across the UK after it was discovered late last year.
The initial strain of Omicron, known as B.1.1.529 has now developed into a further three strains, WHO warns.
BA.1 has now spread to 171 different nations the health organisation said while sub-variants BA.2 and BA.3 are less common.
BA.2 is under investigation after 400 cases were recorded in the UK.
BA.3 has even fewer cases and initial studies in Denmark suggest there is no increased risk of hospitalisation.
UKSHA warn we will continue to see new variants
Dr Meera Chand, Covid-19 Incident Director at UKHSA, said: “It is the nature of viruses to evolve and mutate, so it’s to be expected that we will continue to see new variants emerge as the pandemic goes on.
“Our continued genomic surveillance allows us to detect them and assess whether they are significant.
“So far there is insufficient evidence to determine whether BA.2 causes more severe illness than Omicron BA.1, but data is limited and UKHSA continues to investigate.”
Prof Francois Balloux, Professor of Computational Systems Biology and Director, UCL Genetics Institute, UCL, added: “Viruses tend to evolve fairly fast with different strains constantly acquiring mutations over time. SARS-CoV-2 is no exception to this pattern, with each lineage acquiring two mutations a month on average.”