What Is Herd Immunity to the Novel Coronavirus? Are We Chasing a Dream?

SARS-CoV-2 When considering how the COVID-19 pandemic caused by the novel coronavirus might end, herd immunity is one of the first concepts that comes to mind. Daily cases have approached 500 thousand, and with autumn the pandemic's effects have begun to be felt most severely worldwide; herd immunity is therefore being discussed again at a time of limited resources. On October 19, an article on this subject was published in JAMA Network and attracted attention worldwide. One of its lead authors is a pioneering scientist in her field, Dr. İnci Yıldırım (her biography is available at the end of the article).

Dr. İnci Yıldırım is well known for her work on childhood vaccines. She is now applying her experience to developing an effective COVID-19 vaccine.

What Is Herd Immunity?

Herd immunity, also known as population immunity or community protection, means protecting people susceptible to an infection when enough individuals in a community have acquired immunity. The chart below helps explain herd immunity:

Herd immunity means that immune individuals protect people who have not yet encountered the virus by breaking the chain of transmission. This comes from individual immunity acquired through vaccination.

The term herd immunity was defined more than a century ago. In the second half of the 20th century, its use became more widespread through the need to define goals for immunization and vaccination programs, discussions about eradicating diseases and influential analyses of vaccination programs. However, herd immunity without vaccination had not been considered until the COVID-19 pandemic we are experiencing; the main reason for considering or advocating herd immunity without vaccination now is that our available measures against the novel coronavirus (testing, isolation, masks) are limited and weak.

The eradication of smallpox and reductions in measles and pneumococcal pneumonia are successful examples of vaccine-induced herd immunity.

Understanding Herd Immunity Better

The herd immunity threshold is defined as the proportion of individuals in a community who have acquired immunity and can no longer participate in the transmission chain.

If the proportion of immune individuals in a community exceeds this threshold, the existing outbreak will end and transmission of the pathogen (disease-causing organism) will be interrupted.

In the simplest model, the herd immunity threshold depends on the basic reproduction number (R₀; the average number of other people infected by one infected person in a completely susceptible community).

A highly transmissible pathogen such as measles will have a high R₀ (12-18), requiring a high proportion of the community to be immune to reduce sustained transmission. For severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), since the beginning of the outbreak, R₀  has been estimated to range between 2 and 3. Assuming no population immunity and that all individuals are equally susceptible and infectious, the herd immunity threshold for SARS-CoV-2 is expected to range from 50% to 67% without any intervention. 

Figure: Herd Immunity Thresholds / Proportions for Different Diseases

Duration of Protection

For both naturally acquired and vaccine-induced immunity, the durability of immune memory is critical to population-level protection and maintaining herd immunity; in other words, how long will immunity last in infected or vaccinated people?

Long-lasting immunity to measles, chickenpox and rubella has been acquired through infection as well as vaccination. With seasonal coronaviruses, long-lasting immunity has either not been observed or has been short-lived. For infections producing temporary immunity, the pool of susceptible individuals soon increases without a vaccine, and outbreaks recur. An effective vaccine and vaccination program can maintain herd immunity (even if periodic vaccination is required), and an outbreak can be ended quickly as long as the community maintains the necessary levels.

The Role of Heterogeneity — Socially Active People

Nominal (current) herd immunity thresholds assume random mixing among individuals in a community. However, everyday life is more complex; individuals do not mix randomly, and some interact more than others. Empirically validated network models show that individuals with higher levels of interaction become infected earlier in an outbreak. This may contribute to slowing infection transmission before the nominal herd immunity threshold is reached. However, uncertainty remains about the exact effect of heterogeneity in social mixing on herd immunity against SARS-CoV-2.

T-Cell Cross-Reactivity

T cells are important mediators of immunity (particularly adaptive immunity). Recent research suggests that immunity developed against other coronaviruses may provide relative protection against the novel coronavirus. T-cell cross-reactivity may provide sterilizing immunity, as opposed to reducing disease severity (for example, the host neither harbors nor transmits the infection).

Infection-Based Herd Immunity (Without Vaccination) as Policy

An infection-based herd immunity approach (for example, allowing high-risk groups to become infected while "separating" susceptible groups) has been proposed to reduce SARS-CoV-2 transmission (such as allowing young people to become infected while trying to protect older people). However, this strategy is fraught with risks!

For example, even with the most modest infection fatality rates, a new pathogen will result in substantial mortality because most, if not all, of the population will not have immunity to it. Separating high-risk groups is impractical because infections initially spreading among people with low mortality rates may spread to those with high mortality rates. Moreover, there has been no large-scale example of success with deliberately infection-based herd immunity to date.

There are only rare examples of herd immunity acquired through ongoing infection. The most recent and well-documented example concerns Zika virus in Salvador, Brazil. Early in the COVID-19 outbreak, as other European countries entered lockdown in late February and early March 2020, Sweden chose not to recommend lockdown. Some local experts and journalists initially described this as herd immunity: Sweden would do its best to protect the most vulnerable, but also aimed to observe the number of infections with the goal of acquiring genuine herd immunity. Sweden abandoned this strategy after seeing the grim situation in late March; most universities and high schools closed, travel restrictions were introduced, working from home was encouraged and bans on groups larger than 50 were enacted. Far from acquiring herd immunity, seroprevalence (the proportion of infected people in a community) in Stockholm, Sweden, was reported to be below 8% in April 2020. Despite much higher mortality than its neighbors, it did not even approach herd immunity; this was recorded as a chilling outcome!

The USA has a population of approximately 330 million. Based on the World Health Organization's (WHO) estimated infection fatality rate of 0.5%, around 198 million people in the USA would need immunity to reach a herd immunity threshold of approximately 60%, leading to several thousand additional deaths. Assuming infection-acquired immunity lasts 2 – 3 years and 10% of the population has been infected so far, infection-based herd immunity is not realistic for controlling the outbreak at this point. SARS-CoV-2 vaccines will help reach the herd immunity threshold, but the effectiveness and coverage of the vaccine or vaccines remain to be seen.

Conclusion

In conclusion, naturally acquired herd immunity cannot be expected for the novel coronavirus; it appears possible only through active vaccination. Herd immunity is an important defense against outbreaks and has been successful in regions with sufficient vaccination rates. However, finding an effective and safe vaccine will not be enough to achieve herd immunity; another requirement will be vaccinating most of the population.

Who Is İnci Burçin Yıldırım?

İnci Yıldırım (MD, PhD, MSc) is an Associate Professor in the Departments of Infectious Diseases and Global Health, and Pediatrics at Yale School of Medicine. She is also the Medical Director of Pediatric Transplant Infectious Diseases.

After graduating from Hacettepe University Faculty of Medicine in 1999, she completed residency training in Pediatrics at Yale University and an infectious diseases fellowship at Boston Children's Hospital / Boston Medical Center in Boston. She completed a master's degree in epidemiology and is currently a doctoral student at Boston University School of Public Health.

She received the Massachusetts Infectious Diseases Society's Maxwell Finlandiya Award for Excellence in Research and the Edward H. Kass 2015 Clinical Excellence During Fellowship Award. Dr. Yıldırım's research focuses on the epidemiology of vaccine-preventable diseases, tuberculosis screening strategies, particularly pneumococcal infections, and cost-effectiveness.

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