With more transmissible forms of the coronavirus causing case numbers to rise in some countries, experts have said that the Covid-19 pandemic is far from over.
Now, they are threatening to have a similar effect in many other nations as they outcompete other forms of Omicron.
The concerns over a new wave of cases comes at a time when a study was released this week indicating that the Omicron variant is better able to evade immunity caused by previous coronavirus infections.
It means people who caught Covid during the first wave would receive no boost to their immune response if they subsequently catch Omicron.
Scientists at Imperial College London, who studied triple-vaccinated people, further found that an Omicron infection offered little protection to catching it again, and that the variant was "not a good booster of immunity".
“Having had Omicron doesn’t stop you [from] having it again, which suggests people might get repeated infections,” said Dr Andrew Freedman, an expert in infectious diseases at Cardiff University in the UK.
“Omicron is a weaker virus or less severe. It will probably increase in the winter. The big unknown is whether we will get a variant that is as transmissible but more virulent.”
In an online briefing document published this week, the European Centre for Disease Prevention and Control (ECDC) said the growth advantage for BA.4 and BA.5 — compared with the dominant Omicron variant, BA.2 — was “probably due to their ability to evade immune protection against infection induced by prior infection and/or vaccination”.
The organisation said that “countries should remain vigilant for signals of BA. 4 and BA. 5 emergence and spread”.
While BA.4 and BA.5 caused a rise in case numbers in South Africa during April and May, the peak was much smaller than the country’s previous four waves of Covid-19 infections. The increase in deaths was also much less severe.
In South Africa, it appeared to be the case that hospital admissions associated with Covid-19 increased simultaneously with infections, said Paul Hunter, a professor in medicine at the University of East Anglia.
“Normally, hospitalisations peak about two weeks after the peak in infection,” he said. “If they peak together, most of these hospitalisations are people going into hospital with Covid, not because of Covid.
“There wasn’t much indication BA.4 and BA.5 were causing much hospitalisations because of Covid.”
15 most-recovered tourism markets: in pictures
BA.5 was blamed for a surge in cases in Portugal in May in which there was also an increase in the number of deaths in the country, although this was smaller than previous peaks.
The UK, for example, is probably “at the start of another wave”, Dr Hunter said, most likely driven by BA.4 and BA.5. There are concerns that other countries may experience the same as a result of the easily transmitted forms of the coronavirus.
“The growth advantage reported for BA.4 and BA.5 suggest that these variants will become dominant in the whole European Union/European Economic Area, probably resulting in an increase in Covid-19 cases in the coming weeks,” the ECDC briefing document said.
Although case numbers are at risk of increasing because of BA.4 and BA.5, Dr Hunter said he did not think tougher control measures were necessary.
“Most of these infection-control measures, as has been known for decades, only delay infections,” he said. “Going into lockdown won’t actually prevent infections; it will just delay them.
“Sometimes, delaying infections is what you need if you have got a vaccine coming.”
The risk of delaying infections now, he said, was that it would delay them until people’s immunity from previous vaccination had waned.
The exception lies with people who are particularly vulnerable, who he said were advised to continue to wear a mask, as this could, for example, reduce the severity of disease if they became infected.
Bharat Pankhania, a senior clinical lecturer at the University of Exeter, said the new wave in infections meant people should ensure they were fully vaccinated.
It may also require, he suggested, tweaks to the vaccines to take account of the emerging forms of coronavirus.
“Covid is a lot more dangerous than flu, full stop,” Dr Pankhania said. “Covid has been more dangerous by many magnitudes. There is no comparison.”