Decoded: Why Delta variant of Covid-19 is making herd immunity harder

Many countries are counting on vaccines to build sufficient immunity in their populations so that SARS-CoV-2, the virus that causes Covid-19, isn’t able to find enough people to infect, causing transmission to eventually stop. But even in countries with a high proportion of people inoculated with highly effective vaccines, it’s unclear if it’s possible to reach the so-called herd immunity threshold anytime soon.

Will vaccines eliminate Covid-19?

There’s considerable uncertainty about that. One scientific paper calculated that if a vaccine could provide a lifelong, fail-safe shield against infection with SARS-CoV-2, it would need to reach 60-72 per cent of people to establish herd immunity. But if a vaccine is only 80 per cent effective at preventing any infection, 75-90 per cent of people would need to be immunised — a high bar. The Covid vaccines in use today have been shown to offer 50-97 per cent protection against becoming sick, but it’s mostly unknown how well they prevent people from getting an infection without symptoms that could still be passed on. The rapidly spreading delta variant, which is about twice as infectious as the original strain, is weakening vaccine effectiveness too. Another question is the duration of protection. The shorter it turns out to be, the higher the rates of immunisation required to establish herd immunity. Vaccines might not have to do all the work to get there: Some people who’ve already had the virus will be immunised against it, though not as well as those who’ve been inoculated.

How do variants of the virus factor in?

The more the coronavirus circulates, the more opportunity it has to mutate in ways that enhance its ability to evade the immunity people have gained. Over the past year, such variants have spread globally, leading to new surges in cases. Studies from the UK have shown delta, discovered in India in October, is more resistant to vaccines than the alpha variant that emerged in England in late 2020, particularly in people who have received just the first of two doses. Scientists say the vaccines should still work at stopping severe disease in the vast majority of cases, but some shots may be less effective at protecting against mild infections. Inoculations may need to be updated periodically to maintain their efficacy.

Why is delta especially problematic?

Vaccines have been shown to reduce the concen­tration of virus particles, or viral loads, in the airways of individuals who become infected, lowering the probability of them transmitting their infection to others. Delta, though, is linked with viral loads that are over 1,200 times higher in ne­w­­ly infected individuals compared with the orig­­i­nal coronavirus strain — a feature that helps del­ta spread as easily as chickenpox.