Last year, Covid-19 saw a surge in popularity worldwide. Governments touted the possibility of “herd immunity,” which would allow the virus to stop spreading exponentially if enough people were protected. This is now looking more like a dream.

It was believed that the pandemic would subside and then fade away once a significant portion of the population (perhaps 60% to 70%) had been vaccinated. But new variants like delta, which are more transmissible and been shown to evade these protections in some cases, are moving the bar for herd immunity near impossibly high levels.

Widening spreads of the virus in countries like the U.S., U.K., and other nations that have been already infected by it are being triggered by Delta. These countries have presumably some natural immunity and have had their vaccination rates increase to more than 50%. It is also affecting countries that have managed to eradicate the virus almost completely, such as Australia and China.

This month, the Infectious Diseases Society of America estimated that delta had pushed the threshold for herd immunity to well over 80% and possibly close to 90%. Anthony Fauci, a public health official, has caused controversy by shifting the goalsposts in the last year. This increased the number of people who require protection before they reach herd immunity. The supply and vaccine hesitancy of many countries mean that they won’t be able to reach the original numbers.

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“Will it be possible to create herd immunity?” Greg Poland, director, Vaccine Research Group, Mayo Clinic Rochester, Minnesota, said that it is very unlikely.

He said that even a 95% vaccination rate wouldn’t be enough to achieve it. It is a race to the bottom between ever more transmissible variants that can evade immunity and immunization rates.