TWO new Covid strains have been designated as 'variants of concern' amid signs that they can dodge immune protection, including reinfecting people who previously had Omicron infections.
The variants - known as BA.4 and BA.5 - were first detected in South Africa in January and February respectively, and have since become the dominant strains in the country.
It is the first time since Omicron emerged at the end of November that a new Covid variant has been classified as a variant of concern.
According to the World Health Organisation, BA.4 has been detected in at least 16 countries and BA.5 in 17 countries. Both have been identified in small numbers in the UK.
BA.5 has been making up an increased proportion of cases in Portugal in recent weeks, with the Portuguese National Institute of Health estimating that the strain already accounted for approximately 37% of positive cases by May 8. It is expected to become Portugal's dominant variant by May 22.
Scottish Government epidemiologists previously warned that holidaymakers returning from abroad this summer were likely to seed a new Covid wave.
The European Centre for Disease Prevention and Control (ECDC), which upgraded BA.4 and BA.5 from variants under investigation (VUI) to variants of concern (VoC) late on Friday, said preliminary studies "show a significant change in antigenic [immune response] properties of BA.4 and BA.5 compared to BA.1 and BA.2, especially compared to BA.1".
The original Omicron strain which spread in the UK in the run up to Christmas is known as BA.1, but it has since been overtaken by the sublineage BA.2 which drove the most recent Covid wave in the UK.
South Africa experienced a rapid rise in cases again in mid-April
The ECDC said that the BA.5 is estimated to have a "growth advantage" of around 13% over BA.2; the growth advantage of BA.4 has not yet been quantified.
However, in both cases their ability to spread faster than BA.2 is believed to be driven by "their ability to evade immune protection induced by prior infection and/or vaccination, particularly if this has waned over time".
Watch: Two new Omicron sub-variants responsible for increase in Covid cases in South Africa
There is no indication at this stage that they cause more severe disease.
The ECDC said that "limited available data" from laboratory studies "indicate that both BA.4 and BA.5 are capable of escaping immune protection induced by infection with BA.1 [original Omicron]" such that individuals who are unvaccinated but previously recovered from an Omicron infection "are unlikely to be protected against symptomatic infection with BA.4 or BA.5".
Portugal's case rate has been on the rise since mid-April, driven by BA.5
Vaccinated individuals appear to have better protection, based on in vitro studies using blood plasma from vaccinated people, but this is also likely to wane.
Overall, the ECDC predicts that BA.4 and BA.5 will drive fresh Covid waves in Europe over the summer.
It states: "The presence of these variants could cause a significant overall increase in Covid-19 cases in the EU/EEA in the coming weeks and months.
"The overall proportion of BA.4 and BA.5 in the EU/EEA is currently low but the high growth advantages reported suggest that these variants will become dominant in the EU/EEA in the coming months.
"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.
"However, as in previous waves, if Covid-19 case numbers increase substantially, some level of increased hospital and ICU admissions is likely to follow.
"ECDC encourages countries to remain vigilant for signals of BA.4 and BA.5 emergence.
"Early variant detection critically relies on sensitive and representative testing and genomic surveillance, with timely sequence reporting."
In Scotland, the number of patients in hospital with Covid has been falling steadily, in line with sampling by the Office for National Statistics. All patients are still tested for Covid using PCRs, but the ONS is currently reducing the number of households it tests each week for routine surveillance
It comes after the World Health Organisation's chief executive, Tedros Adhanom Ghebreyesus, warned that a substantial reduction in Covid testing globally was leaving the agency "increasingly blind to patterns of evolution and transmission" of the virus.
Scotland ended routine PCR testing - which is required for sequencing of variants - on May 1 as part of a UK-wide policy in favour of "targeted surveillance" mainly involving hospital patients.
The ECDC said "continued close epidemiological and vaccine effectiveness monitoring is essential in order to rapidly detect signals of increased SARS-CoV-2 [the virus which causes Covid] circulation or risk of severe disease among vaccinated individuals".
It added: "If such signals emerge, a second booster may be considered for some or all adults 60 years and older and for other vulnerable groups".
In Scotland, a second Covid booster is already being offered to all patients with health conditions that increase their risk, and to all adults over 75.
A separate new Covid strain - BA.2.12.1 - has so far been detected in 23 countries, and is spreading rapidly in the United States. It is already the dominant strain across New York and New Jersey.
During an online Q&A, the WHO’s technical lead on Covid, Maria Van Kerkhove, said she expects to see an increase in case detection of BA.2.12.1 worldwide due to its higher growth rate over BA.2, but stressed that there is also no evidence that it causes more severe disease.
She urged governments across the world to closely monitor BA.2.12.1, BA.4, and BA.5, adding: “We talk to government all the time about the need to maintain the surveillance systems so that we can track this, we can trace it, and we can assess it in real time.”