Herd immunity to stop COVID-19?

When do we reach immunity?

The point at which we reach herd immunity is mathematically related to the germ’s propensity to spread, expressed as its reproduction number, or R0. The R0 for the coronavirus is between 2 and 2.5, scientists estimate, meaning each infected person passes it to about two other people, absent measures to contain the contagion.

To imagine how herd immunity works, think of coronavirus cases multiplying in a susceptible population this way: 1, 2, 4, 8, 16, and so on. But if half the people are immune, half of those infections won’t ever happen, and so the spreading speed is effectively cut in two. Then, according to the Science Media Centre, the outbreak simmers along like this instead: 1, 1, 1, 1 … The outbreak is snuffed out once the infection rate is less than 1.

The current germ’s rate of spread is higher than that of the ordinary flu, but similar to that of novel emergent influenzas that have occasionally swept the globe before. “That is similar to pandemic flu of 1918, and it implies that the end of this epidemic is going to require nearly 50% of the population to be immune, either from a vaccine, which is not on the immediate horizon, or from natural infection,” Harvard University epidemiologist Marc Lipsitch told a gathering of experts on a video call.

The more infectious a virus is, the more people need to be immune for us to achieve herd immunity. Measles, one of the most easily transmitted diseases with an R0 over 12, requires about 90% of people to be resistant for unprotected people to get a free ride from the herd. That’s why new outbreaks can start when even small numbers of people opt out of the measles vaccine.

Similarly, if the coronavirus spreads more easily than the experts think, more people will need to get it before herd immunity is reached. For an R0 of 3, for example, 66% of the population has to be immune before the effect kicks in, according to the simplest model.

Whether it’s 50% or 60% or 80%, those figures imply billions infected and millions killed around the world, although the more slowly the pandemic unfolds, the greater the chance for new treatments or vaccines to help.

The newest epidemiological models developed in the UK now recommend aggressive “suppression” of the virus. The basic tactics being urged would be to isolate sick people, try to reduce social contacts by 75%, and close schools. Those economically costly measures could continue for many months.

“Suppressing transmission means that we won’t build up herd immunity,” says Azra Ghani, the lead epidemiologist on the new model of the outbreak from Imperial College London. The trade-off of success is “that we are driving it down to such a low level that we have to keep those (measures) in place.”

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Source: the technology review