What difference does it make when you get your second AstraZeneca jab? | SMH AU |

The Delta variant has acquired mutations that slightly change the shape of its spike protein - the microscopic harpoon it uses to enter our cells - meaning some antibodies that would have gummed up earlier variants slide right off Delta. These changes appear to significantly cut the effectiveness of both AstraZeneca and Pfizer's vaccines. Those drop-offs in effectiveness are most pronounced after a single dose of either vaccine.

"You used to get really good levels of protection against Alpha and Beta [variants] with a single jab. That's very much changed now," says Associate Professor Nathan Bartlett, head of the viral immunology and respiratory disease group at the Hunter Medical Research Institute at the University of Newcastle. "And that's the issue with a vaccine that was designed based on a virus that was circulating almost two years ago now."