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An infectious diseases expert from Monash University explains what makes the delta variant of COVID-19 so challenging. It all comes down to R numbers.

This number expresses the rate of virus spread of each unique strain in each country. R numbers affect public health decisions about everything from lockdowns to border closures, to vaccine responses.

Delta has changed things

Monash Postdoctoral Research Fellow Dr Michael Lydeamore says, ‘The delta variant has really changed the game. Analyses from modelling groups across the UK suggest that the delta variant is between 30 to 60 per cent more infectious than the alpha variant, which itself is about 30 per cent more infectious than what was circulating this time last year.

‘There is no evidence that the serial interval – the time between symptoms of one case and the next case – is any shorter for the delta variant. The emerging evidence does suggest this variant is easier to catch – perhaps requiring more “fleeting” contact than before.

‘But, once someone has the virus, they seem to take just as long to develop symptoms, become infectious, and recover at the same rate as any other strain of COVID-19’, said Dr Lydeamore.

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New and improved?

‘The effective transmission number, or the “R number”, will be higher compared to last year. Just how high, we’ll have to wait and see,’ he said. ‘These things don’t just depend on the strain of the virus, but also the local conditions – and Australia has so far had a lower R compared to international estimates,’ said Dr Lydeamore.

‘Although everyone wants R to be less than 1, the amount it’s below 1 is also important when it comes to reducing case numbers. Last year, we saw lockdowns reduce R down to 0.5 – so, 20 cases becomes 10, then becomes 5, then becomes 2. But now, because of the increased infectiousness, R may only reduce to 0.8 – so, 20 cases becomes 18, then 12.

‘It means that lockdowns, and these broad restrictions, may need to last longer – like they did in Victoria a few weeks ago,’ he said.

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R numbers driving lockdowns

‘The higher infectiousness, and the resulting longer times returning to very small case numbers, is one of the reasons why jurisdictions have been so fast with the implementation of their lockdowns. We’ve seen two examples – one in Victoria, and one in New South Wales – of longer than expected restrictions,’ said Dr Lydeamore.

‘That’s obviously not ideal, so everyone is locking down at the first hint of a case,’ he said.

‘Both the AstraZeneca and the Pfizer vaccines are highly effective at reducing infectiousness. They remain our best way to combat the pandemic. Global studies are now saying that these vaccines work well against the delta variant too, which is good news.

‘No vaccine is perfect, of course, but we’re seeing around the world that the benefits far outweigh the risks of contracting COVID-19,’ said Dr Lydeamore.