An effective pandemic response requires clear rules and thresholds that tell us how to judge COVID risk. But that doesn't mean the virus always follows the script.
"While we talk about antibodies declining after five months or so, that's an average … for some people, it's going to be two months, for others it's going to be 12 months," says Stuart Tangye, an immunologist with the Garvin Institute who is working to understand why some people get severely ill with COVID, while others don't get it at all.
Firstly a person has to breathe in enough of the virus to reach what Stuart Turville, an associate professor at the Kirby Institute, calls a "threshold of viable virus". By now we're familiar with COVID's effective reproduction rate — what scientists call the "Reff" or "R number" — which measures how many people a positive person will typically pass the virus on to. For an outbreak to be controlled, this number needs to be below one.Inside the body, at a cellular level, there's another "reproductive rate" going on.
This is in line with what Professor Michael Good, one of Australia's top infectious disease experts, calls the 20-80 rule: "Eighty per cent of transmission is in general caused by 20 per cent of those who are infected," he toldResearch is currently underway to understand why this is.
Kate Simpson's two adult children both tested positive for COVID-19. But despite spending time with them indoors, she never contracted the virus.
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