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Understanding the Impact of Vaccination on Herd Immunity

Understanding the Impact of Vaccination on Herd Immunity

February 22, 2022

The COVID-19 pandemic has had a profound impact on our daily activities, shuttering schools and businesses, and claiming a half million lives in the US to date. Unfortunately, the pandemic has disproportionately affected vulnerable populations, including seniors.

Fortunately, science has prevailed with vaccines approved by the FDA for emergency use to help society gain an edge in this fight. As vaccines are delivered to priority groups (e.g., essential workers, seniors, immunocompromised), there is a renewed hope for a return to a “new normal.” It is, therefore, critical that we accelerate herd immunity via vaccination, which requires education, informed consent, and distribution.

Our public health system continues to improve vaccine logistics with recent reports showing 1.5M vaccinations per day. However, we can benefit from improved education to bolster vaccination rates. For instance, a recent CDC study indicates that 30-60% of frontline healthcare workers at nursing homes are choosing to delay vaccination. Various groups across the country seem discomforted in historical vaccine development. From an epidemiological perspective, the question is what impact could delays in vaccination have on herd immunity and our ability to return to a “new normal.”

Understanding Herd Immunity 

Herd immunity occurs when a substantial proportion of the population becomes immune to a pathogen via direct infection or vaccination (we prefer vaccination!). Once a majority of the population is immune to the COVID-19 virus, the risk of transmission from one person to another is reduced, providing protection to individuals who are not yet immune. We examine the factors that determine the time until we reach herd immunity in the United States. 

For those interested in the math, we break down the variables to calculate herd immunity below. TL;DR the faster we can immunize a greater proportion of the population, the sooner we achieve herd immunity and can consider laxing social distancing measures.

Herd Immunity Threshold is the percentage of the population needed to be immune to achieve herd immunity. This percentage depends on the infectivity rate (i.e., “R0” aka R-naught) of the pathogen, which for COVID-19 is estimated to be between 2-3 . We will assume an R0 of 3 for our calculations, especially in light of the more recent COVID-19 variants with higher infectivity levels. This means that someone who contracts COVID-19 will spread the infection, on average, to 3 additional people. In contrast, the flu has an R0 around 1.2.

Base Prevalence is the percentage of the population that has already become immune to the virus through either direct infection or vaccination. As of the date of this article, there have been over 28 million COVID-19 cases in the US. With a US population of about 330M, over 8% of Americans have contracted COVID-19. In addition, nearly 60 million vaccine doses have been administered in the US with 12% of the population having received at least one dose and 5% of the population having received both doses.

Monthly Infection Rate is the rate of the population that acquires immunization through direct infection of the virus each month. The monthly infection rate has varied significantly over the past 6 months from 50,000 to 300,000 new daily infection rates. Currently, the number is hovering around 75,000 daily new infections or 2.25 million cases per month (i.e., 0.68% of the US population monthly).

Estimating Time to Herd Immunity

The US has ordered 600M vaccine doses to be delivered by July 2021, sufficient to immunize the majority of the country. Assuming vaccine logistics can handle the volume, let’s assume complete vaccination by August 2021. The remaining factor will be vaccine confidence and the resulting proportion of the population that agrees to immunization. Below, we explore the impact of vaccine participation on achieving herd immunity.

Low Vaccine Participation of 40%

Moderate Vaccine Participation of 50%

High Vaccine Participation of 60% or Greater

Thankfully, we have a path to herd immunity this summer assuming high participation and continued public health logistical support. There are a few caveats with these estimates, including that two vaccine doses are required to achieve 90% efficacy. New strains are evolving via mutation which may have higher infectivity or may reduce vaccine efficacy. Sustained immunization from vaccines is still unknown and being evaluated in clinical trials; it’s possible booster shots may be necessary to maintain sufficient antibodies over time.

The Importance of Vaccination

As we can see from the estimates above, each incremental increase in vaccine participation in our communities has a dramatic impact on the speed at which we can safely achieve herd immunity to protect our most vulnerable populations. Vaccination coupled with prudent social measures (e.g., masks, social distancing, hand washing) is paramount to protecting ourselves, our family, and our neighbors. Education and policy reform are key to the US expediently reaching herd immunity. 

At Tembo Health, we are seeing the disproportionate impact of COVID-19 on senior communities and hope we can pull together as a society. In short, please get vaccinated!

Supporting Senior Community Vaccination

Tembo Health is supporting senior communities to vaccinate their residents and staff with education and automated digital consents. Our physicians have held webinars and created informational materials to educate staff, residents, and families on the benefits of vaccination. In addition, our emergency medicine physician team is available 24/7 to manage personal health questions and any symptoms related to the vaccine. Finally, Tembo’s HIPAA-compliant digital vaccine consent forms have streamlined the process where family or healthcare proxies often need to sign off on medical decisions, thereby maximizing participation. 

Contact our team at hello@tembo.health to learn more about our telemedicine and clinical support services.


This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment and does not constitute medical or other professional advice. All calculations provided in this article are using publicly available data and should not be taken as absolute. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. 


  1. Early COVID-19 First-Dose Vaccination Coverage Among Residents and Staff Members of Skilled Nursing Facilities (CDC)
  2. What is Herd Immunity and How Can We Achieve It With COVID-19? (Johns Hopkins)
  3. Fauci says herd immunity possible by fall, ‘normality’ by end of 2021 (Harvard Gazette)
  4. Estimating the COVID-19 R number: a bargain with the devil? (The Lancet, Chris T Bauch)
  5. Coronavirus Vaccine Hesitancy in Black and Latinx Communities (Langer Research Associates)
  6. Some Healthcare Workers Refuse to Take COVID-19 Vaccine, Even with Priority Access (LA Times)
  7. How Is The COVID-19 Vaccination Campaign Going In Your State? (NPR)
  8. Report: Many US nursing home staff decline first COVID shots (Associated Press) 
  9. Pfizer-BioNTech COVID-19 Vaccine Information Page (CDC)
  10. Moderna COVID-19 Vaccine Information Page (CDC)

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