Coronavirus: Double-strength Delta strain to dominate

The Indian strain of Covid-19 will almost certainly become Australia’s most common variant if case numbers increase, experts say.

Volunteers prepare to bury a body of a Covid-19 coronavirus victim in Chennai, India. The Delta variant emerged in India around October last year and has now spread to 100 countries. Picture: AFP

The Delta strain of Covid-19 will almost certainly become the dominant strain of coronavirus in Australia if case numbers increase, following the variant’s dominance in the UK over the original Wuhan virus and the Alpha variant.

The Delta variant, also known as B.1.617.2, is about 50 per cent more infectious than the Alpha strain that first emerged in Britain last year, according to the latest data from Public Health England. The Alpha strain was about 40 per cent more infectious than the original Wuhan strain, making the Delta strain almost twice as infectious as the original wild type of the virus.

The Delta variant emerged in India around October last year and has now spread to 100 countries. It has emerged as the dominant strain of the virus in the UK owing to its increased transmissibility, with 12,431 cases of the Delta variant detected in Britain since the strain first arrived from India in January. Delta variant case numbers rose by 5472 in the week up to June 3, much faster than any of the other variants of concern.

According to Public Health England, one person infected with the Delta strain is likely to pass the virus on to 12.4 per cent of their contacts. Somebody infected with the Alpha strain was likely to pass the virus on to 8.2 per cent of their close contacts.

Medical staff at the Arcare aged-care centre in Maidstone, Melbourne, on Monday. Picture: David Crosling

However, those numbers are averages, and the number of people an infected case will pass the virus on to is highly variable, given that some people pass the disease on to many people and others pass it on to nobody.

Victoria’s chief health officer, Brett Sutton, said on Monday that several recent positive Victorian cases had passed the Delta strain on to 75 per cent of their household contacts.

Professor of epidemiology and preventive medicine at the Alfred Hospital Allen Cheng, who chairs the Australian Technical Advisory Group on Immunisation, has said the reproductive number of the Delta variant is about five, compared to the original Wuhan strain which had an R0 of around 2.5. That means on average one person infected with the Delta strain will pass the virus on to five people.

Deakin University epidemiologist Catherine Bennett said it was highly likely the Delta strain would become dominant in Australia, with more overseas travellers arriving infected with that variant.

Commuters at Euston station in central London. The Delta strain became the dominant coronavirus variant in the UK owing to its increased transmissibility. Picture: AFP)

“It’s almost certain that there is increased transmissibility that will rapidly lead to it becoming the dominant UK variant,” Professor Bennett said.

“Following the patterns in other countries, particularly the UK with the precursor Alpha variant being overtaken by Delta, we might expect to see the same in Australia.”

Three-quarters of cases of the Delta strain in Britain have been among unvaccinated people, with only 3.7 per cent of cases in people who were fully vaccinated.

Emerging evidence from Britain suggests people infected with the Delta variant are more likely to be hospitalised.

Public Health England’s latest technical briefing on SARS-CoV-2 variants of concern under investigation states: “Early evidence ­suggests there may be an increased risk of hospitalisation for Delta compared to Alpha although more data is needed for us to have more confidence in that finding.”