“Herd Immunity” is an important term and is a concept that is critical to understand as it pertains to human and animal health,” says Gerald Stokka, North Dakota State University Extension veterinarian and livestock stewardship specialist.
“Individual immunity is important to us all as immunity is critical to our very survival,” Stokka says. “We are constantly exposed to potential disease-causing organisms, and yet the vast majority of people are asymptomatic (show no obvious signs or symptoms of disease) or have only mild symptoms for a short period of time.”
A good example of the importance of individual immunity is the practice of vaccinating horses to protect them against the disease caused by West Nile virus. The West Nile virus is transmitted to horses through the mosquito. This virus cycles between mosquitoes and bird populations, with horses and humans becoming infected when bitten by an infected mosquito.
Horses, humans and other mammals are known as dead-end hosts because they cannot infect other like species. Preventing the mosquito bite is difficult and thus protection through vaccination is important. The vaccination of mosquitoes and wild birds is logistically impossible at this time; thus, the need exists for an effective vaccine in the horse population.
The three objectives of vaccination are to:
•Induce protection against disease
•Reduce susceptibility of an individual to infection
•Reduce infectivity (shortens time and severity) after the occurrence of an infection
“Although estimates vary widely, the growing consensus of the reproductive number for COVID-19 appears to be between two and three,” says Paul Carson, professor of practice in NDSU’s Department of Public Health.
The goal with vaccination or exposure (new cases) is to have this number less than one.
“When this occurs, not enough new cases are generated to propagate the infection, and an outbreak will soon be extinguished,” Carson says. “Therefore, the goal of herd immunity is the reduction of infection or disease in the susceptible segment of a population as a result of immunity (through vaccination and/or natural infection) in a substantive proportion of the population.”
The chances of exposure and infection decrease with an increasing number of individuals who are immunized. This results in a decrease in the transmission of the pathogen within the group such that new infections become controlled or extinct.
The threshold for which we can expect to see herd immunity is directly related to the contagiousness of the pathogen (the reproductive number). Using the example of COVID-19, with a reproductive number of 2.5, the proportion of the population needing to be immune to halt further spread would be 0.6 (60% of the population needs to be immune to stop the spread).