The concept of herd immunity is at the heart of global vaccination efforts and discussions about next steps in fighting the Covid-19 pandemic and bringing back economies.
For the pandemic to stop, the coronavirus has to run out of susceptible hosts to infect. Herd immunity occurs when enough people in a population develop an immune response, either through previous infection or vaccination, so that the virus can’t spread easily and even those who aren’t immune have protection.
To reach herd immunity for Covid-19, public-health authorities estimate that around 60% to 70% of a given population would need to develop an immune response to the virus. Some epidemiologists and mathematicians now say herd effects might start to kick in before that point, at perhaps closer to 50%, suggesting potential protection could be achieved sooner.
Still, infectious-disease experts adamantly warn against the notion of trying to reach herd immunity to the coronavirus without a vaccine, as the costs on human life would be staggering and it likely wouldn’t happen soon, if at all.
Even with a vaccine, there will still be barriers to achieving herd immunity. “It’s a continuous process,” said Saad Omer, director of the Yale Institute for Global Health. “You could start seeing [an effect] before that threshold, but the other issue is there might still be outbreaks at a smaller level.”