13 May 2020 Author : Betânia Ferreira Herd immunity to against a contagious infectious disease in a population (human or animal) occurs when a sufficiently large part of that population is immune (protected) against that disease and it does not contribute its spreading. Herd immunity can be achieved in two ways. How is herd immunity achieved? There are two ways for a population to have herd immunity to a disease: By mass vaccination of the population against the disease; By natural immunity developed by an important part of the population, after being infected. For diseases against which vaccines exist, vaccination is the appropriate and most effective way to achieve herd immunity. Many serious diseases are now eradicated or controlled thanks to population vaccination programs. For example: Herd immunity through vaccination played a decisive role in the eradication of smallpox in the 1970s; Measles outbreaks in recent years in several countries were due to low vaccination coverage, a consequence of non-compliance with national vaccination programs (which are universal, but not mandatory). Whenever there is a vaccine, immunity acquired naturally, allowing the disease to spread freely should not be an alternative. Several reasons why natural immunity is not as effective as vaccination to achieve a herd immunity state are, for example: To be immune, it is necessary to be infected first; Developing the disease can have serious health risks associated; Many people may not know if they have been infected or are immune to the disease. There are also infectious diseases for which vaccination allows herd immunity to be obtained, but for which natural immunity does not allow it, because they are not contagious from person to person. This is the case, for example, of tetanus. What characterizes herd immunity When in a population there is herd immunity to a disease, the spread of this disease is limited because the majority of people who are exposed to the infection, by a virus, a bacterium, or another biological agent, have defenses that allow them not to become infected and develop the disease. So, in a herd immunity situation : The transmission chains are interrupted; The likelihood of someone not immune to contact with someone infected it reduced; More vulnerable groups, who may not be immune, such as babies and children, elderly, pregnant women, and people with a compromised immune system, are more protected because the contact with infected people is less likely. For each disease, the percentage of the population that must be immune to safely decrease or stop contagion, that is, to achieve a herd immunity situation, is called the herd immunity threshold. In many cases the value is in the range of 80% -95%. Why with COVID-19 not allowing the infection to spread freely was not an option to achieve herd immunity since there is no vaccine? In the case of COVID-19, a new disease, whose characteristics and consequences are still unknown, not adopting containment measures and allowing it to spread freely, would have caused the contagion, the number of cases of serious illness with hospital care need and mortality to be much higher. The incapacity of health services to respond to the number of cases that would occur would also contribute to the severity of the cases and mortality associated with the disease. Thus, currently, the best measures to prevent the occurrence and transmission of the disease are: For most people: keep as much as possible at home, physical distance, frequent hand washing, individual protection in recommended situations; Quarantine in cases of suspected exposure; Isolation of confirmed cases of illness. At present, herd immunity through natural immunity is not an option to control COVID-19. It is also not possible through vaccination because there is no vaccine yet. Infectious and contagious diseases Infectious diseases are caused by biological agents, for example, viruses, bacteria, fungi, among others. Contagious diseases are diseases in which there is contagion from person to person, either directly or indirectly, for example through contamination of contaminated objects. All contagious diseases are infectious, but not all infectious diseases are contagious. Meet the internal medicine team from Hospital da Luz network. The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item.
