Delta was recognised as a SARS-CoV-2 variant of concern in May 2021 and has proved extremely difficult to control in unvaccinated populations.
Delta has managed to out-compete other variants, including Alpha. Variants are classified as “of concern” because they’re either more contagious than the original, cause more hospitalisations and deaths, or are better at evading vaccines and therapies. Or all of the above.
So how does Delta fare on these measures? And what have we learnt since Delta was first listed as a variant of concern?
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How contagious is Delta?
The R0 tells us how many other people, on average, one infected person will pass the virus on to.
Delta has an R0 of 5-8, meaning one infected person passes it onto five to eight others, on average.
This compares with an R0 of 1.5-3 for the original strain.
So Delta is twice to five times as contagious as the virus that circulated in 2020.
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What happens when you’re exposed to Delta?
SARS-CoV-2 is the virus that causes COVID-19. SARS-CoV-2 is transmitted through droplets an infected person releases when they breathe, cough or sneeze.
In some circumstances, transmission also occurs when a person touches a contaminated object, then touches their face.
One person infected with Delta infects, on average, five to eight others.
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Once SARS-CoV-2 enters your body – usually through your nose or mouth – it starts to replicate.
The period from exposure to the virus being detectable by a PCR test is called the latent period. For Delta, one study suggests this is an average of four days (with a range of three to five days).
That’s two days faster than the original strain, which took roughly six days (with a range of five to eight days).
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