Core concept: herd immunity is an important and often misunderstood public health phenomenon.
PNAS May 25, 2021 118 (21) e2107692118; https://doi.org/10.1073/pnas.2107692118
Commentary
To achieve "herd immunity" against SARS-CoV-2, it is estimated that 70-85% of the population would need to be vaccinated in the United States. But what is really the real number? So there is too much uncertainty, even a year after the pandemic has occurred.
And while we cannot provide a clear threshold, there is concern that we will face difficulties in reaching a high setting. The basis of herd immunity has been described as the proximity of immune individuals protecting others who are still susceptible. This means that the presence of enough immune people in the population is a protective barrier against the spread of the viral pathway.
Mathematical calculations
Mathematically, the threshold for herd immunity can be easily calculated as 1 - 1 / R 0. Calculating from this number, it is higher than the seasonal influenza R 0 of 1.3 (11), but much lower than the measles R 0 of somewhere between 12 and 18. Thus, based on the overall average of three new cases per infected person, the herd immunity threshold for COVID-19 is 1 - (1/3), or 0.67.
This result means that 67% of the population needs to be immune in order for SARS-CoV-2 to wane, but in the real world it is not that simple.
Even with the calculation that gave the answer of 67%, it is assumed that the population will randomly mix and a person carrying SARS-CoV-2 will transmit it to about 3 people for a susceptible population.
Thus, the result of the calculation, despite the uncertainties, can be thought of as "transient herd immunity. If the assumption that vaccination provides complete immunity is true, then the chance of contracting the virus and transmitting it to others is zero.
Then, if 67% of the population is fully vaccinated, about two-thirds of infections should be reduced, and the route of infection should be one person instead of three. One of the unknowns, however, is whether it is possible to acquire immunity to SARS-CoV-2 through vaccines or other means. This is a question that remains to be answered.
Therefore, vaccination is recommended in order to obtain herd immunity, but even if many people are vaccinated, what does herd immunity actually represent? This does not address the question of.
What does herd immunity mean?
Herd immunity can be seen as a kind of herd immunity because social distancing, even just wearing a mask, can lower R0 below 1, or it can be seen as herd immunity because changes in people's behavior can lead to fewer cases. However, this is not to say that a group has reached herd immunity. Herd immunity explains that a group develops immunity through a combination of vaccination and natural infection.
Conclusion
The concept of herd immunity can be too slippery.
That is why the best way to tell how a pandemic will end? is an ongoing debate. Vaccination to achieve herd immunity, protection by natural immunity, etc. It is easy to set a goal of 67% man-made, but the fact is that these numbers are not absolute.
That said, it is also likely that the "end" of the pandemic does not mean the end of the virus, explaining that we may need another way to explain the end of the pandemic and the return to "normal".
