Medicosnext
Vaccines greatly reduce the risk of infection by working with the body’s natural defenses to safely develop immunity to disease (Centers for Disease Control and Prevention, USA).
Vaccination protects people from getting the disease and passing on the pathogen, thus breaking any chains of transmission. And, vaccination of a significant segment of the population results in herd immunity, which WHO’s chief scientist, Dr Soumya Swaminathan, describes as, “having a barrier of people who are protected, who break that chain of transmission. So, you don’t need every single person in the area, in the population, to necessarily be protected.” In other words, herd immunity is the indirect protection conferred to a population where the majority has natural or acquired immunity to an infection, which is possible either through a large part of the population getting infected or vaccinated.
Generally, a certain percentage of the population (threshold proportion) must be susceptible to a disease in order for it to spread. If the percentage of immune population is greater than this threshold, the spread of the disease will decline (herd immunity threshold).
Although herd immunity can be achieved by allowing a disease to spread naturally through any segment of the population, the World Health Organization (WHO) supports achieving it through vaccination to avoid the obvious collateral damage arising from the former. Besides other reasons, one aim of achieving herd immunity is to protect those who cannot get vaccinated for some reason or another, for example, because of allergic reaction susceptibility or compromised immune system. Achievement of herd immunity through vaccination has successfully controlled deadly diseases like smallpox, polio, diphtheria, rubella, etc.
A substantial part of a population needs to be vaccinated to achieve effective herd immunity.
The question is: what percentage of the population needs to get vaccinated in order to achieve herd immunity? It is different for different diseases, but the more contagious the disease, the
greater the percentage of population that needs to be vaccinated. For instance, measles, which is a highly contagious disease, requires 95% of a population to be vaccinated to achieve herd immunity, while the percentage is 80% for polio. What about COVID-19? The answer is not yet determined, although Dr. Swaminathan says, “We think that it needs at least 60 to 70% of the population to have immunity to really break the chain of transmission.”
Researchers believe that the reproduction number (R0) for COVID-19 is between 2 and 3, that is, one person can infect two to three others. This means that 50% to 67% of the population would need to be resistant before herd immunity is achieved.
Among some of the hurdles for achieving herd immunity through vaccination is the fact that revaccination may be required due to waning protection of the vaccine over time. And, it is also a possibility that people may not get all of the shots required to be completely protected. Then, there are many people who don’t want to be vaccinated for various reasons; for example, some may have religious objections, others may be afraid of possible risks, and some may just not trust the effectiveness of a vaccine. If there are significant numbers of such individuals in a community, the proportion of vaccinated people will fall below the herd immunity threshold, resulting in rapid spread of the disease.
According to WHO, less than 10% of the population in most countries have been infected with COVID-19, thus making most populations susceptible to the virus. Because it is a new virus, research is still ongoing to find out more about the disease, including the subject of immunity.
While most COVID-19 infected people develop an immune response within the first few weeks, it is unknown how strong or lasting the immune response is. Furthermore, immunity could differ in different people, with reports of some being infected for a second time.
Whether the strength and length of immune response depends on the type of infection— asymptomatic, mild, or severe—is another issue being looked into, with researchers finding that even people without symptoms appear to develop an immune response. As with most diseases, immunity has been found to decline over time in the case of other coronaviruses like the common cold, SARS-CoV-1, and Middle East Respiratory Syndrome (MERS).
References:
1. WHO, https://bit.ly/3nBpz9k)
2. Mayo Clinic, https://mayocl.in/3q0Ufma
