Herd Immunity Is Something That Can’t Arrive Soon Enough
November 2, 2020 at 6:35 p.m.
By Max [email protected]
Herd immunity occurs when a significant portion of a population becomes immune to an infectious disease and the risk of spread from person to person decreases. Those who are not immune are indirectly protected because ongoing disease spread is small. The proportion of a population who must be immune to achieve herd immunity varies by the disease. A disease like smallpox, for example, is very contagious, and requires more than 95% of the population to be immune to stop sustained disease transmission and achieve herd immunity. Less is known about COVID-19. Herd immunity can also be achieved by vaccination. The book Understanding Numbers provides this explanation:
“Suppose you have vaccinated a proportion p of the community, so these people are now immune to the disease. This means that a proportion 1-p is still susceptible. The basic reproduction number R gives the number of people a sick person infects on average in a totally susceptible population. Since after vaccinating only a portion, 1-p of the population are still susceptible, the reproduction number is only a portion, 1-p, of what it was in a totally susceptible population: the basic reproduction number of R turns into an effective reproduction number R x (1-p), in order for the disease to fizzle out. Doctors would like the effective reproduction number to be less than1, so R x (1-p) < 1. By rearranging , the proportion vaccinated must be at least 1 – 1/R < p.
“In other words, if you follow the math the disease dies out when you vaccinate a proportion of at least 1 – 1/R of the population. For a basic reproduction number of 2, you need vaccinate 1 – ½ or one half the population. If R is 3 (and that may be the case for the coronavirus and influenza) you should vaccinate 1 – 1/3 or 2/3 of the population.“ (Note: most of the studies estimated the SARS-CoV-2 R value to be in the range of 2 to 3.)
Herd immunity may be achieved either through infection and recovery or by vaccination. Vaccination creates immunity without the need to contract the disease and thus an approved vaccine is critical to stemming the outbreak. Herd immunity also protects those who are unable to be vaccinated, such as newborns and people who are immunocomprimised. Communities with lower vaccine coverage may have outbreaks of vaccine preventable diseases because the proportion of people who are vaccinated is below the necessary herd immunity threshold.
Achieving herd immunity through infection relies on enough people being infected with the disease and recovering from it, during which they develop antibodies against future infection. According to the Journal of the American Medical Association (JAMA), in some situations, even if a large proportion of adults have developed immunity after prior infection, the disease may still circulate among children. In addition, antibodies from a prior infection may only provide protection for a limited duration. People who do not have immunity to a disease may still contract an infectious disease and have severe consequences of that disease even when herd immunity is very high. Herd immunity reduces the risk of getting a disease but does not prevent it for nonimmune people.
As mentioned above, there is currently no effective vaccine against SARS-CoV-2. It is not yet known if having this disease confers immunity to future infection, and if so, for how long. The duration of immune memory is a critical factor in determining population level protection and sustaining herd immunity. A large portion of people would likely have to be infected and recover to achieve herd immunity; however, this situation could overwhelm the health care system and lead to many deaths and complications.
In a second article in JAMA, the authors studied herd immunity as it is related to SARS-CoV-2 control. They reported that based on the U.S. population of 330 million, the World Health Organization estimated of an infection fatality rate of 0.5%, about 198 million individuals in the U.S. are needed to be immune to reach a herd immunity threshold of approximately 60%, which would lead to several hundred thousand additional deaths. Assuming that less than 10% of the population has been infected so far, with an infection-induced immunity lasting two to three years, infection induced herd immunity is not realistic to control the pandemic.
Final Thoughts
In light of the controversy over herd immunity, it remains wise to continue efforts of prevention including keeping distance between yourself and others, washing hands often with soap and water or sanitizer with 60% alcohol, and to wear a mask in public places where it is difficult to avoid close contact with others.
Max Sherman is a medical writer and pharmacist retired from the medical device industry. His new book “Science Snippets” is available from Amazon and other book sellers. It contains a number of previously published columns. He can be reached by email at [email protected].
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Herd immunity occurs when a significant portion of a population becomes immune to an infectious disease and the risk of spread from person to person decreases. Those who are not immune are indirectly protected because ongoing disease spread is small. The proportion of a population who must be immune to achieve herd immunity varies by the disease. A disease like smallpox, for example, is very contagious, and requires more than 95% of the population to be immune to stop sustained disease transmission and achieve herd immunity. Less is known about COVID-19. Herd immunity can also be achieved by vaccination. The book Understanding Numbers provides this explanation:
“Suppose you have vaccinated a proportion p of the community, so these people are now immune to the disease. This means that a proportion 1-p is still susceptible. The basic reproduction number R gives the number of people a sick person infects on average in a totally susceptible population. Since after vaccinating only a portion, 1-p of the population are still susceptible, the reproduction number is only a portion, 1-p, of what it was in a totally susceptible population: the basic reproduction number of R turns into an effective reproduction number R x (1-p), in order for the disease to fizzle out. Doctors would like the effective reproduction number to be less than1, so R x (1-p) < 1. By rearranging , the proportion vaccinated must be at least 1 – 1/R < p.
“In other words, if you follow the math the disease dies out when you vaccinate a proportion of at least 1 – 1/R of the population. For a basic reproduction number of 2, you need vaccinate 1 – ½ or one half the population. If R is 3 (and that may be the case for the coronavirus and influenza) you should vaccinate 1 – 1/3 or 2/3 of the population.“ (Note: most of the studies estimated the SARS-CoV-2 R value to be in the range of 2 to 3.)
Herd immunity may be achieved either through infection and recovery or by vaccination. Vaccination creates immunity without the need to contract the disease and thus an approved vaccine is critical to stemming the outbreak. Herd immunity also protects those who are unable to be vaccinated, such as newborns and people who are immunocomprimised. Communities with lower vaccine coverage may have outbreaks of vaccine preventable diseases because the proportion of people who are vaccinated is below the necessary herd immunity threshold.
Achieving herd immunity through infection relies on enough people being infected with the disease and recovering from it, during which they develop antibodies against future infection. According to the Journal of the American Medical Association (JAMA), in some situations, even if a large proportion of adults have developed immunity after prior infection, the disease may still circulate among children. In addition, antibodies from a prior infection may only provide protection for a limited duration. People who do not have immunity to a disease may still contract an infectious disease and have severe consequences of that disease even when herd immunity is very high. Herd immunity reduces the risk of getting a disease but does not prevent it for nonimmune people.
As mentioned above, there is currently no effective vaccine against SARS-CoV-2. It is not yet known if having this disease confers immunity to future infection, and if so, for how long. The duration of immune memory is a critical factor in determining population level protection and sustaining herd immunity. A large portion of people would likely have to be infected and recover to achieve herd immunity; however, this situation could overwhelm the health care system and lead to many deaths and complications.
In a second article in JAMA, the authors studied herd immunity as it is related to SARS-CoV-2 control. They reported that based on the U.S. population of 330 million, the World Health Organization estimated of an infection fatality rate of 0.5%, about 198 million individuals in the U.S. are needed to be immune to reach a herd immunity threshold of approximately 60%, which would lead to several hundred thousand additional deaths. Assuming that less than 10% of the population has been infected so far, with an infection-induced immunity lasting two to three years, infection induced herd immunity is not realistic to control the pandemic.
Final Thoughts
In light of the controversy over herd immunity, it remains wise to continue efforts of prevention including keeping distance between yourself and others, washing hands often with soap and water or sanitizer with 60% alcohol, and to wear a mask in public places where it is difficult to avoid close contact with others.
Max Sherman is a medical writer and pharmacist retired from the medical device industry. His new book “Science Snippets” is available from Amazon and other book sellers. It contains a number of previously published columns. He can be reached by email at [email protected].
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