SINGAPORE - Covid-19 cases around the world are rising, with Singapore going through a wave of infections driven by Omicron sub-variants BA.4 and BA.5. Average infections over a seven-day period here climbed to 8,664 as of noon on July 24, from 5,523 about a month ago, according to the Ministry of Health website.

BA.5, in particular, has fuelled a rise in Covid-19 infection globally. According to a World Health Organisation report published two weeks ago, the variant was behind 52 per cent of cases sequenced in late June, up from 37 per cent in one week.

There is concern that these Omicron sub-variants may lead to more severe infections. Preliminary laboratory research shows that they have a mutation in the spike protein, LR452, that allows them to bind better to lung tissue, compared with earlier Omicron variants BA.1 and BA.2, which usually infect the cells of the upper respiratory tract.

But experts here tell The Straits Times there is no cause for concern for now.

The Ministry of Health also said on Monday that there is "currently no clear evidence that Omicron variants cause more severe disease".

Associate Professor Hsu Li Yang, an expert on infectious diseases at the NUS Saw Swee Hock School of Public Health, says: "This is partly because such a distinction would need careful and large-scale clinical studies to distinguish. Also, partly because global immunity to Covid-19 is such a patchwork of vaccinated and/or infected individuals, it will be hard to confirm if there really is a difference in relation to earlier variants.

"We should not expect it to be different as it is ultimately the same virus."

Most people, he adds, will still experience upper respiratory tract infection symptoms including sore throat, runny nose and cough.

"Some may experience loss of taste and smell to varying extents, but these symptoms were also reported for the earlier variants," he says.

Dr Leong Choon Kit, a family physician at Mission Medical Clinic in Serangoon, says he has not observed that the newer BA.4 and BA.5 variants affect the lungs more than the throat. "In theory, all the Covid-19 variants affect the lungs, but the earlier variants of Covid-19 (such as Delta) seem to affect the lungs more than the newer variants," he notes.

Several laboratory studies have found that compared with the Delta variant, the Omicron variant does not infect cells deep in the lung as readily as it does those in the upper airways.

For instance, a study published in a journal titled Med in March found that Omicron infection in hamsters may result in strong upper respiratory tract infection, but less severe lower respiratory tract clinical disease, compared with previous variants.

This could be why the Omicron variant is more easily transmissible.

Professor Ravindra Gupta, a virologist at the University of Cambridge, who co-authored one of the studies, said in a Nature journal article: "If the variant lingers in the upper airways, viral particles might find it easy to hitch a ride on material expelled from the nose and mouth, allowing the virus to find new hosts."

While experts say there is no difference in symptoms among the Covid-19 variants, the waning effects of the vaccine is something to watch out for.

Says Prof Hsu: "Immunity against infection by these viruses is not lifelong, with the risk of reinfection rising after four to six months, especially with the emergence of new variants.

"A steep rise in infections will result in a rise in hospitalisations and deaths, despite the protection offered by vaccinations and prior infections."

Even though vaccines currently available are designed against the ancestral variant, Dr Leong Hoe Nam, an infectious diseases physician at Rophi Clinic, says these still offer excellent protection for the vulnerable.

"It reduces the risk of hospitalisation by five- to tenfold until the new variant-specific vaccine becomes available. Protect yourself against the death threats of the new variants by getting boosted in your vaccination," he says.

Overall, Professor Paul Tambyah, president of the Asia Pacific Society of Clinical Microbiology and Infection, expects the Covid-19 virus to evolve and become less of a threat.

When viruses evolve, they become more easily transmissible and less deadly, he says. "This happened with the deadly Spanish flu in 1918, which became the dominant strain of seasonal influenza for nearly 40 years. It happened with H1N1 influenza A in 2009 and has happened with every known human virus. The experience with the original strain followed by Delta and then Omicron shows that the Covid-19 virus is no different," he adds.