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  • Rob Swanda, PhD

Covid-19 Boosters: The Shield to our Armor

Updated: Jan 12, 2022

Words & Video by Dr. Rob Swanda

Nearly instantly after receiving a vaccine our cells react by launching an immune response to create memory B and T cells and protective antibodies against the desired pathogen. This is our armor being assembled, piece by piece. Collectively, as more people suit up, we form a barrier around our families, friends, and communities to prevent the pathogen from invading. However, the armor isn’t always perfect and over time it can rust or chip away. But, that doesn’t mean the battle is over. While treatments are being developed as weapons to aid in destroying the pathogen, we can protect ourselves further by picking up a shield. A shield in the form of a vaccine booster.

As the SARS-CoV-2 virus continues to impact millions of people around the world, getting vaccinated remains the single greatest protective benefit against the risks of severe illness and death associated with Covid-19. In the United States, everyone aged 12 years+ are already eligible to receive a vaccine against Covid-19, while emergency use authorization (EUA) for the 5–11 year-old cohort is expected to be approved momentarily. As of 25 October 2021 (and under the current CDC definition) just over 57% of the United States population and 67% of those aged 12 years+ are fully vaccinated against Covid-19. While this protection is incredibly helpful, these percentages do leave gaps in our barrier. A gap found in almost every other person we encounter. It’s these gaps that weaken our defenses against SARS-CoV-2 by allowing the virus to continue entering our homes and infecting those we care for.

Outside of the clinical trials, worldwide data supports initial laboratory trials of how the vaccines were anticipated to perform. In the United States, the unvaccinated are currently at 6X greater risk of testing positive for Covid-19, 12X greater risk of requiring hospitalization, and >11X greater risk of dying from Covid-19 compared to their fully vaccinated peers. Unfortunately, these gaps also allow SARS-CoV-2 to linger in our communities and further mutate. Then, when a high enough amount of virus is present it has a chance to strengthen itself and find cracks in the armor of those who thought they were protected.

With a variant having higher transmissibility and infectivity (eg: Delta) in the community and the first vaccines administered in the United States more than 10 months ago, it’s understandable that a booster program was soon going to be front and center as an effort to save more lives. Think of your antibodies waning like a leaky faucet. Slowly over time that dripping will reduce your water supply, but probably will never run your well dry. A booster serves to tighten the bolts on your faucet. However, everyone may not need this additional boost of protection. Scientists and medical professionals have conveyed a variety of people to determine who would benefit most from added protection.

Depending on your age and circumstances in the United States, those aged 18+ are eligible for a Covid-19 booster. Vaccine boosters for all three vaccine types used in the United States are approved for use following successful clinical trials and subsequent EUA amendments. As of 25 October 2021, eligibility is understood as depicted in the chart below.

Chart: US Covid-19 Booster Eligibility as at 25 October 2021 -
Chart 1: US Covid-19 Booster Eligibility as at 25 October 2021

The Covid-19 vaccine boosters are largely identical to the initial shots administered. Currently, the only adjustment in the United States is the dosage of Moderna’s Covid-19 booster which has been lowered from 100mcg to 50mcg. It is not yet clear how that may affect the geometric mean fold rise in the binding and neutralizing antibody assays, however it is clear across multiple studies that using an mRNA booster does provide increased level of protection regardless of which vaccine you were originally fully vaccinated with.

A recent study conducted in Israel examined two groups of individuals: 1. those vaccinated >5 months ago, and 2. those vaccinated <5 months ago. Results showed that those who were vaccinated >5 months ago had an increased risk of contracting SARS-CoV-2 compared to those more recently inoculated. Very few vaccinations of any kind offer a single-dose lifetime protection against their target disease or infection, especially during a pandemic where the pathogen of interest is abundantly present and circulating. Therefore, it is important that we continue on a path of prioritizing protection for those who are vulnerable in our population. With the recommended boosters, our front-line defense against further infection can be strengthened.

Over the past several months as more clinical trials in United States were conducted using thousands of participants (across a variety of Covid-19 vaccine manufacturers), no new safety concerns were identified. These trials specifically included organ transplant recipients and severely immunocompromised individuals. What is compelling is the additional approval allowing for the mixing of vaccines as the booster dose. This leaves most of us to essentially choose which booster we would like to receive. Additionally, this removes any issue with trying to find a location that only has the original manufacturer you received, creating a more streamlined process and easier access to higher protection. There is already preliminary research from multiple countries confirming the mRNA type vaccines are best placed to deliver a higher antibody response compared to their non-mRNA type peers. The findings for an NIH preprint study have been summarized below (see Chart 2), which can aid you in determining which booster type may be best based on your initial series.

Chart: How do Covid-19 vaccine boosters compare? -
Chart 2: How do Covid-19 vaccine boosters compare?

For those outside the United States who received the Oxford AstraZeneca vaccine (a similar adenovirus viral vector type as the Johnson & Johnson) studies have confirmed largely comparable outcomes that using an mRNA type shot as a booster can elicit increased protection. As an example, a study in Sweden of more than 721,000 individuals, all who received an initial AstraZeneca vaccine received a booster shot of either the AstraZeneca, Pfizer, or Moderna. Those who were given the all-AstraZeneca series were provided 50% protection against infection. This increased to 67% when the administered booster was Pfizer and 79% for Moderna.

Interestingly, the Swedish study went one-step further by matching participants with an unvaccinated person of the same gender, age, and locality. Researchers found that the unvaccinated were 3.5X more likely to contract SARS-CoV-2 than those who had received vaccination. The difference to the earlier mentioned 6X factor for the United States is likely attributable to Sweden’s higher vaccination rates (see Chart 3), leading to less virus and transmission overall in their communities.

Chart: Share of people vaccinated against COVID-19 as of Oct 23, 2021 in US and Sweden -
Chart 3: Share of people vaccinated against COVID-19 as of Oct 23, 2021 in US and Sweden
Chart: Number of COVID-19 patients in hospital per million people - US vs Sweden as of Oct 17, 2021 -
Chart 4: Number of COVID-19 patients in hospital per million people - US vs Sweden as of Oct 17, 2021

This is further evidenced by the rates of hospitalization (see Chart 4), as of 17 October 2021 and adjusted for population, the United States had a rate of 154 hospitalizations per million people compared to Sweden’s rate of 21 per million people.

If in your location only one type of vaccine or booster is available, you should consider taking it. The best vaccine or booster is always likely to be that which is available. Additionally, if you are concerned about side effects from getting your vaccine you may decide to stick with the original manufacturer. For example, if you had no side effects from the Pfizer vaccine, then you may decide to continue with Pfizer as you are more familiar with how your body could react.

You should seek the advice of your physician(s) or other qualified healthcare provider(s) with any questions or concerns you may have regarding your circumstances, medical history, condition(s) and which vaccination may be best for your own unique personal situation.

Video: COVID-19 Booster Shots & Mixing Vaccines

Data sources:

Administration, U. F. (2021, 10 20). Coronavirus (COVID-19) Update. Retrieved from U.S. Food & Drug Administration:

Centers for Disease Control and Prevention. (2021, 10 25). COVID-19 Vaccinations in the United States. Retrieved from Centers for Disease Control and Prevention:

Centers for Disease Control and Prevention. (2021, 10 25). COVID Data Tracker. Retrieved from COVID-19 Vaccinations in the United States:

Centers for Disease Control and Prevention. (2021, 10 22). COVID Data Tracker Weekly Review. Retrieved from Centers for Disease Control and Prevention:

Centers for Disease Control and Prevention. (2021, 08 26). Delta Variant: What We Know About The Science. Retrieved 10 23, 2021, from Centers for Disease Control and Prevention:

Flynn, H. (2021, 10 21). COVID-19 vaccine ‘mix and match:’ Benefits of an mRNA second dose. Retrieved 10 22, 2021, from Medical News Today:

medRxiv. (2021, 10 15). Heterologous SARS-CoV-2 Booster Vaccinations – Preliminary Report. medRxiv, 28 pages.

Retrieved from:

O'Connor, D. (2021, 09 27). The message from Israel is clear: Covid booster shots should be standard. Retrieved 10 24, 2021, from The Guardian:

Our World In Data. (2021, 10 23). Coronavirus (COVID-19 Vaccinations). Retrieved from Our World In Data:

The Lancet. (2021, 10 17). Effectiveness of heterologous ChAdOx1 nCoV-19 and mRNA prime-boost vaccination against symptomatic Covid-19 infection in Sweden: A nationwide cohort study. The Lancet, 7 pages.

Retrieved from

The New England Journal of Medicine. (2021, 09 15). Protection of BNT162b2 Vaccine Booster against Covid-19 in Israel. NEJM, 8 pages.

Retrieved from


Author: Rob Swanda, PhD of

Title: Covid-19 Boosters: The Shield to our Armor

Date first published: 25 October 2021


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