There has been a great deal of discussion around the term herd immunity and whether it can be achieved in the current setting of COVID-19. What is herd immunity, and can we count on it to halt the spread of this novel coronavirus? Fine, Eames & Heymann (2011) outline several definitions for herd immunity including: a threshold proportion of immune individuals that should lead to a decline in incidence of infection, or a pattern of immunity that should protect a population from invasion of a new infection. In her blog entitled “What is Herd Immunity?
,” Lisa Bonsall, MSN, RN, CRNP, discusses how “herd immunity, or community immunity, makes it less easy for communicable diseases to spread, especially to those for whom vaccination is contraindicated.”
How does a community achieve herd immunity?
Herd immunity is achieved when a population becomes immune either from previous infection or by vaccination (World Health Organization [WHO], 2020). Individual natural immunity occurs after direct exposure to a pathogen. Upon contracting a viral or bacterial infection, the body develops antibodies to fight the microorganism and memory cells are generated to help protect a person from re-infection in the future. In some diseases like the measles, immunity will last a lifetime, however in other illnesses such as influenza, immunity will only last six months to a year. Individuals recovering from COVID-19 produce antibodies to the virus, but unfortunately, we don’t yet know how long that protective response will last.
When does a community reach herd immunity?
Herd immunity occurs when the number of individuals with immunity reaches a point at which the virus can no longer find susceptible hosts to infect and therefore can no longer spread. The herd immunity threshold depends on transmissibility of the disease and can be calculated using the reproduction number, R0 (“R zero” or “R naught”) which tells us the average number of people that an individual with the virus can infect in a completely susceptible population (Randolph & Barreiro, 2020). The higher the R0, the more contagious the disease and thus the more people need to be resistant to the virus to stop transmission. R0 will depend on population density, population structure and contact rates across demographic groups (Randolph & Barreiro, 2020). While the R0 will vary by city, state and country, scientists believe the R0 for COVID-19 falls between two and three; in other words, one person can infect two to three people (WebMD, 2020). This translates to approximately 50% to 70% of the population (over 200,000,000 Americans) that would have to contract the virus and recover in order to achieve herd immunity in the U.S. alone. Exposing a significant proportion of the population is unrealistic and ethically questionable as COVID-19 has caused serious illness resulting in excessive hospitalizations, stress on the healthcare system and above all a significant loss of life.
The second and preferred way to establish herd immunity is through vaccination which provides immunity without causing severe illness or side effects. When enough people get vaccinated to meet the herd immunity threshold, individuals who aren’t able to get vaccinated such as newborns or those with compromised immune systems are then protected (Mayo Clinic, 2020). Vaccines have been successful in controlling contagious diseases such as smallpox, polio, diphtheria, measles, mumps, rubella, varicella zoster and influenza, to name a few (Amanna & Slifka, 2018).
The biggest challenge is that we are still months away from an approved vaccine to fight this novel coronavirus. No one vaccine will be 100% effective in every single individual. Efficacy will depend on the strength and duration of the immunity acquired from the vaccine (Randolph & Barreiro, 2020). Once developed, approved, and manufactured, distribution of the vaccine to billions of people worldwide will be the next hurdle. Other issues involve poorly administered vaccines outside of recommended schedules (Fine et al., 2011) and immunity from vaccination may fade over time, requiring a booster. Viruses are also known to mutate, which necessitates the reconfiguration of vaccines based on new viral strains. Finally, some people are reluctant to get vaccinated either because of religious reasons, fear of vaccine side effects, uncertainty about the safety and benefits, lack of time or money, or a sense that it is inconvenient.
While natural herd immunity is not the answer to halting the spread of COVID-19, even if we have an approved vaccine by early 2021, we will continue to face many challenges in eradicating the virus. As a society, we need to be patient, stay the course and individually take responsibility to mitigate the transmission of this disease. Remember to avoid mass gatherings, maintain physical distancing
, wear a face mask in public spaces and when you can’t physically distance, wash your hands often, avoid touching your eyes, nose, and mouth, and stay home if you are sick. Be sure to follow your local public health guidelines.