The danger of Delta

By Stuff reporter

The Whole Truth: Covid-19 Vaccination | By Stuff senior writer Nikki Macdonald.

When an Aussie tourist who wined and dined his way around Wellington was found to be infected with Covid-19, there was one question on everyone's lips - did he have the Delta strain of the virus?

Why? Because it’s a doubly-infectious juggernaut pushing out other SARS-CoV-2 mutations and causing case surges everywhere from Israel to Britain to the United States.

First identified in India in October 2020, the World Health Organisation labelled Delta a variant of interest in April. It was upgraded to one of four variants of concern in May, indicating it could be more infectious, cause more severe disease, or be better at evading vaccines or public health measures.

Now present in 96 countries, Delta is on track to become the globally dominant variant.

The biggest concern with Delta is that it’s much more efficient at spreading between people. One Sydney case is believed to have been infected by a “scarily fleeting” contact in a mall.

Some estimates suggest Delta is 60 per cent more infectious than the Alpha variant that swept across Britain, and twice as infectious as the original Wuhan strain.

That might not sound that bad, until you consider that viruses spread exponentially.

Remember the R0 value - the average number of people an infected person spreads the virus to, without public health controls?

The R0 for the original Wuhan strain is about 2.5, the Alpha strain is about 3.75 and the Delta strain is about 5.

So for the Wuhan strain, three rounds of spread from a single person would result in about 16 people being infected. For the Delta strain, that same number would be 125.

That increased infectiousness also makes it harder to reach population immunity - the vaccination level beyond which any outbreaks naturally die out. Recent modelling found 83 per cent of Kiwis would need to be vaccinated to reach population immunity with the Alpha strain, but that rose to 97 per cent for Delta.

Delta also seems to behave differently. In Britain, more young people than previously have been infected. Early evidence suggests it might also increase the risk of hospitalisation, but more data is needed.

It also seems to produce slightly different symptoms. A British survey found a headache, sore throat and runny nose were among the most common symptoms, making it harder to differentiate from a common cold.

The good news is the Pfizer vaccine still works well against Delta, but it’s critical to get both doses. An early study (not yet peer-reviewed) found the Pfizer vaccine was 88 per cent effective at preventing symptomatic disease after the second dose, but only about 33 per cent effective after the first dose.

Data from Israel suggest the vaccine might only prevent infection in about two thirds of those vaccinated, but it prevents severe disease in more than nine out of 10 vaccinated people.