Herd Immunity
To better understand herd immunity (aka population or community immunity), looking at the origin story of the term - how it came about, how it has been used in the past, key assumptions - is necessary. Today we are going to take a deep dive, starting in the 1800s, to understand what herd immunity is and the benefits it can provide. While vaccines do contribute to the creation of a group/population/herd of individuals who are protected from the disease despite not being vaccinated, the take-away message for today is simple —
Everyone (12+) needs to be vaccinated.
We are in the midst of a pandemic, and a full-on community vaccination effort (participation from everyone who is eligible) is needed to end transmission of the SARS-CoV-2 virus.
In college, I came close to majoring in history. I ended up declaring Environmental Science as my major (which was the best decision for me obviously!) because my favorite professor offered to be my advisor (it made declaring my major as a shy, insecure 19-year-old a stress-free experience). I did earn a minor in history and I do think of myself as an amateur historian, especially when it comes to the history of disease. My interest in history is definitely tied to books - I love reading about history and historical novels (a list of my favorite books is at the end of this post). I spent a lot of time during the past few weeks reading about and researching the history of herd immunity. What is below is the tip of the iceberg, but it provides the needed context for the daily conversations about herd immunity that are happening all around us.
The earliest record of herd immunity dates back to 1840 when William Farr (the father of medical statistics) noted that —
“The smallpox would be disturbed, and sometimes arrested, by vaccination, which protected part of the population.”
Similarly, in 1906 Sir William Hamer wrote —
“The number of transmissions per measles case was a function of the number of susceptibles in the population.”
Both documented that the transmission of an infectious disease (one that spreads from person to person) would slow or even end when the number of individuals susceptible to the infectious agent decreased. When more people are immune (sustained immunity, not merely 90 days post-infection), the virus has fewer opportunities to infect individuals and continue to spread at epidemic rates.
The term HERD IMMUNITY was not coined until 1918 when a veterinarian wrote about an outbreak of “contagious abortions” in sheep and cattle. Side note — he was really talking about a herd of sheep and cattle. Hense the term HERD IMMUNITY. Dr. George Potter wrote this about a bacteria causing spontaneous abortions in cattle in the Journal of the American Veterinary Medical Association in 1918 —
“Abortion disease may be likened to a fire, which if new fuel is not consistently added, soon dies down. HERD IMMUNITY is developed, therefore, by retaining the immune cows, raising the calves, and avoiding the introduction of foreign (suscepticle or infected) cattle.”
The modern definition of HERD IMMUNITY — honestly, there are several definitions used throughout public health and clinical medicine. All three definitions (listed below) are rooted in the ideas noted by Farr, Hamer, and Potter that the risk of infection/disease among susceptible individuals (those without immunity) in a population is reduced by the presence and proximity of immune individuals. Meaning if you surround yourself with people who have immunity, you are unlikely to become ill or contract the disease even if you are susceptible.
The term “herd immunity” was not widely used by the public health or medical communities until relatively recently (the past 40 years or so). Herd immunity is associated with the increasing use of vaccines and discussions of disease eradication programs (think smallpox and polio). Since the 1980s, the public health community has known three important things about herd immunity —
Long-term interruption of disease transmission requires CONTINUAL vaccinations as long as the infection remains endemic in other communities. In terms of COVID-19, vaccinations must continue among those who are unvaccinated and we need to be looking to the future and thinking about booster shots for those of us who were vaccinated in early 2021. Our goal is to interrupt disease transmission - to a disease that prior to November 2020 not a single human being on Earth had immunity to. Herd immunity is not “one and done.” It will be a process and requires the sustained/continual immunity of a large proportion of individuals across the globe.
Many difficulties surround the attainment of herd immunity to eradicate, eliminate, or stop the transmission of disease. Effective community-wide control of infectious diseases will depend on multiple mitigation strategies, including vaccinations, vector control, and efforts to improve nutrition, hygiene, sanitation, and education. To stop the spread of COVID-19 in our communities, not only do we all need to participate (collective action), but we need to participate through various methods of prevention. We need to get vaccinated AND mask AND wash our hands AND be cautious around the most vulnerable. This is about ALL OF US contributing to the work of public health. Cooperation is needed.
At the start of a mass vaccination program, the likelihood of severe disease arising from the vaccine is ORDERS OF MAGNITUDE SMALLER than the risk of serious disease arising naturally. Do you hear that?!?! Getting vaccinated is actually safer than getting COVID-19. Your chances of severe disease, hospitalization, and death are exponentially higher if you contract COVID-19 than if you are vaccinated. The vaccine is safe.
What is the take-away message? Vaccines are safe and effective.
The COVID-19 vaccine was designed to protect the individual who is vaccinated from disease, hospitalization, and death. When enough people are vaccinated against COVID-19, there will be community-wide impacts, namely disease transmission rates will begin to decline. Breaking chains of disease transmission through vaccinations (illustrated above) will decrease the number of cases AND provide additional protection for individuals, including every child under the age of 11 who cannot get vaccinated at this time.
While choosing whether or not to get vaccinated is an individual choice - and please read my letter to my daughter about the vaccine if you are not vaccinated yet - everyone who is vaccinated is contributing to the greater good and improving our community’s health. You being vaccinated not only protects you from becoming ill (and potentially dying), but it is also increasing the number of individuals who are immune and may in fact protect someone who is susceptible to the disease from becoming ill.
The COVID vaccine will protect you and will save other lives.
While you might feel invincible against COVID-19, your neighbor may not be. The vaccine is one way to love our neighbors and create healthy communities for all.
Definitions of herd immunity — The proportion of immune individuals in a population. For example, where I live the proportion of individuals who are fully vaccinated for COVID-19 is approximately 36%. Meaning between 3-4 people out of every 10 are vaccinated.
Threshold proportion of immune individuals that should lead to a decline in the number of new infections. Dr. Fauci says that we will need 70-85% of the population to be vaccinated before the total population (the herd) will be protected from the SARS-CoV-2 virus.
The pattern of immunity that should protect the population from new infection. Meaning that specific demographic and geographic groups with high levels of immunity will be less likely to see new infections. This is linked to the first definition; in my community where the proportion of vaccinated individuals is low, we are likely to see sustained transmission of disease and higher rates of hospitalizations and deaths. Herd immunity is tied to global and local vaccination rates.
Great books! A few recommendations -- And the Band Played On
Beating Back the Devil
House on Fire
Isaac’s Storm
Pox Americana
The Coming Plague
The Great Influenza
The Next Pandemic
The Vaccine Race
Vaccine