The term "herd immunity" is used to describe the phenomenon that individuals who are at risk of an infectious disease are protected by those around them that have immunity against the infection.
The level of herd immunity needed to protect a population is mostly determined by the basic reproduction number (or R0) of an infectious agent. In highly contagious diseases like measles, where a single individual with measles will, on average, infect 15 susceptible others (R0 = 15), approximately 95% of the population will need to be immune to measles to effectively reach herd immunity. Vaccine-induced herd immunity has dramatically reduced the spread of measles, and other infectious diseases, since the mid-20th century.
COVID-19 has an R0 of 3, which means that approximately 60% of the population will need to acquire immunity to infection to protect the entire population. In the absence of a vaccine, the only way that this level of herd immunity can be reached is by having the virus spread through the population and assuming that those that survive have acquired immunity.
I am unaware of any infectious disease where humanity reached herd immunity due to infection and subsequent recovery and immunity. For example, smallpox caused 300 million deaths in the 20th century but was only controlled due to a sustained vaccination campaign, not due to herd immunity through infection.
Nevertheless, some have called for a ‘controlled spread’ of COVID-19 through the population, particularly in younger individuals as they are less likely to die from the infection, in order to reach herd immunity. This hypothetical scenario would then protect the elderly and others who are at a very high risk of dying due to COVID-19. This is an idea that may look good in theory, but is very difficult, if not impossible, to accomplish in practice.
First of all, the infection fatality rate due to COVID-19 in those younger than 65 may be low but is also not nil. A recent study estimated that the infection fatality rate is approximately 0.4% for those aged 55. Controlled spread of COVID-19 would thus still lead to a significant number of preventable deaths in the community. In addition, it will be essentially impossible to effectively shield the elderly, and younger individuals with underlying health conditions, as their self-isolation cannot be complete while the virus spreads through a population. They will sometimes still need to engage with individuals in the community that may potentially transmit the virus thus seeding new outbreaks. Finally, even younger COVID patients are at a risk of complications during infection. These complications may not lead to death but can impact their health significantly for months or possibly even years to come. An estimated 10% of COVID-19 patients experience prolonged illness (more than 3 weeks after diagnosis) and a subset of these patients may exhibit extreme fatigue and cardiological and neurological symptoms for longer periods.
For these reasons, a strategy based around herd immunity through infection should be considered extremely risky. The alternative option is to effectively perform COVID-testing and subsequent isolation of cases and tracing of contacts. Test-trace-isolate, in combination with reducing the number of potential transmission events by working from home if possible, social distancing, frequent handwashing and wearing of masks, have been shown to be effective in reducing the transmission of COVID-19 in a number of countries across the world.