Herd immunity

The term herd immunity was first introduced more than a century ago. The term was later increasingly used in the context of immunisation programmes to refer to the need for sufficient vaccination coverage. This is the prerequisite for protecting a population from an infectious disease. More than any other medical intervention, immunoprophylaxis of infectious diseases has achieved the greatest successes in the history of medicine. Particularly in the case of viral infections for which no effective antidote is yet available, immunoprophylaxis is of fundamental importance. The eradication of smallpox (when on 22.101977 the Somali cook Ali Maow Maalin was released from hospital cured, the world was thus freed forever from the worst scourge of mankind, smallpox. Today, no infectious smallpox viruses exist in the world. Only 2 laboratories in the world still keep lyophilized smallpox viruses) and the continuing reduction in disease incidence in adults who no longer need to be vaccinated after routine immunization of children against Haemophilus influenzae type B and pneumococci are impressive examples of the effect of vaccine-induced herd immunity.

Herd immunity is the naturally or artificially acquired immunity against a pathogen in a defined population (collective protection). Herd immunity protects individuals from infection (individual protection). In particular, it also protects people who cannot be vaccinated for various reasons. A prerequisite for herd immunity is that there is a sufficiently large proportion of immune individuals in a population. A further prerequisite for herd immunity is a homogenous-homonogenous chain of infection (when an infectious disease spreads, it is transmitted from person to person. Only humans are affected). The pathogens must not have any other hosts.

The threshold of herd immunity is defined as the proportion of individuals in a population who can no longer participate in the chain of transmission because they are immune through vaccination or after overcoming disease. If the proportion of immune individuals in a population is above this threshold, current outbreaks dry up. Endemic transmission of the pathogen is interrupted. In the simplest model, the threshold of herd immunity depends on the baseline reproduction number (R0; the average number of individuals infected by an infected individual in a fully susceptible population) and is calculated as 1 - 1/R0. Highly infectious agents such as measles virus or SARS-CoV-2 virus have a high R0 value. To achieve herd immunity, a high proportion of the population (approximately 95% in the case of measles) must be immune. For SARS-CoV-2, the threshold of herd immunity is likely to be between 50 and 67%.

Natural herd immunity: In order to slow the spread of viral infection, herd immunity based on a natural history of infection has been repeatedly advocated. However, such a strategy is burdened with a considerable mortality even with moderate rates of fatal infections (example of the Brazilian metropolis Manaus). For the United States, it is calculated that about 198 million people need to be immune to reach the required herd immunity threshold of about 60 percent. With a SARS-CoV-2 mortality rate of at least 0.5 percent (World Health Organization estimate), this means that several hundred thousand people would die if an infection-induced immunity strategy were adopted.

Sexually transmitted diseases: High-risk behaviors during sexual intercourse lead to high transmission rates of the pathogens in question, making eradication of STDs more difficult. In the case of STDs, the herd effect may extend from one sex to the other.