Herd immunity is a key concept for epidemic control, which states that only a proportion of a population needs to be immune to an infectious agent for it to stop generating large outbreaks. We can achieve herd immunity through two strategies: by overcoming natural infection or through vaccination.
Authors of this article Arnaud Fontanet & Simon Cauchemez state that there is little evidence to suggest that the spread of SARS-CoV-2 might stop naturally before at least 50% of the population has become immune.
So, what would it take to achieve 50% population immunity?
- One of the main problems is we currently do not know how long the naturally acquired immunity to SARS-CoV-2 lasts. The duration of immunity is particularly of concern among those who had mild forms of Covid-19. If we take seasonal coronaviruses as the benchmark, they usually have short-lived immunity.
- We should also check whether it might take several rounds of re-infection before robust immunity is attained against SARS-COV-2. With flu pandemics, herd immunity is usually attained after two to three epidemic waves.
- The cost of reaching herd immunity through natural infection would be very high, especially among individuals who are at a higher risk of developing severe complications. That’s why an effective vaccine presents the safest way to reach herd immunity.
As of August 2020, six anti-SARS-CoV-2 vaccines have reached phase III trials. We can assume that some will become available by early 2021, although their safety and efficacy remain to be established. We wait for further evidence.