Biotechnology What is herd immunity and can it stop the coronavirus? Once enough people get Covid-19, it will stop spreading on its own. But -- -- infected and (if they survive) become immune that the outbreak will fizzle out on its own as the germ finds it harder and harder to find a susceptible host. This phenomenon is known as herd immunity. More on coronavirus -- -- * Our most essential coverage of covid-19 is free, including: What is herd immunity? What is serological testing? How does the coronavirus work? -- -- Those figures aren’t a random guess. They are informed by the point at which epidemiologists say herd immunity should kick in for this particular virus. -- -- particular virus. Last week the herd immunity idea blew up in the headlines after UK prime minister Boris Johnson indicated that country’s official strategy might be to put on a stiff upper lip and let the disease run its -- -- course. The chief science adviser to the UK government, Patrick Vallance, said the country needed to “build up some kind of herd immunity so more people are immune to this disease and we reduce the transmission.” -- -- Yesterday, the prime minister of the Netherlands, Mark Rutte, struck a similar note, saying, “We can slow down the spread of the virus while at the same time building group immunity in a controlled way.” -- But shooting for herd immunity right away would be a disastrous strategy, according to the newest models. That’s because so many people will become severely ill—and a sudden boom in sick people needing -- -- could be spared and lives saved, but ultimately the result could be the same. That is, even if the pandemic is drawn out over time, it may still take herd immunity to bring it to an end. As Matt Hancock, the UK Secretary for health and social care, clarified -- -- As Matt Hancock, the UK Secretary for health and social care, clarified after criticism of the UK government: “Herd immunity is not our goal or policy. It’s a scientific concept.” -- -- policy. It’s a scientific concept.” But what exactly is herd immunity? When enough of the population is resistant to a germ, its spread stops -- -- infection somewhere around 1% (pdf) (that too is uncertain, and the fatality rate of cases rushed to the hospital is higher), we’ve seen evidence for the emergence of herd immunity in other recent outbreaks. Herd immunity In a simple model of an outbreak, each case infects two more, creating an exponential increase in disease. But once half the population is immune, an outbreak no longer grows in size. -- -- about. A Brazilian study found by checking blood samples that 63% of the population in the northeastern beach city of Salvador had already had exposure to Zika; the researchers speculated that herd immunity had broken that outbreak. -- -- broken that outbreak. Vaccines create herd immunity too, either when given widely or sometimes when administered in a “ring” around a new case of a rare infection. That’s how diseases like smallpox were eradicated and why -- -- in some people. For herd immunity to take hold, people must become resistant after they are infected. That occurs with many germs: people who are infected and recover become resistant to getting that disease again, because their -- -- About 80,000 people have recovered from the coronavirus already, and it’s likely they are now resistant, although the degree of immunity remains unknown. “I would be surprised, but not totally surprised, if people did not become immune,” says Myron Levine, an infectious disease -- -- people did not become immune,” says Myron Levine, an infectious disease expert at the University of Maryland. Some viruses, like the flu, do find ways to keep changing, which is why immunity against such seasonal germs isn’t complete. -- -- germs isn’t complete. 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 -- -- 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 -- -- 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 -- -- 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. -- -- 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