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What is Efficacy?

Efficacy is a measure of how well something works. Efficacy numbers of the COVID vaccines appear in print frequently efficacies are 95% for Pfizer, 94.1% for Moderna, 70% for AstraZenica (not yet available in the US as of this writing), and 70% for Johnson & Johnson. So, what do these percentages mean?

The Pfizer efficacy of 95% means that 95% of those vaccinated will develop the desired effect. In other words, if, in an unvaccinated group of 100 persons, 20 develop COVID, then in a Pfizer-vaccinated group of 100 persons, only 1 will develop COVID.

Similarly, the Johnson & Johnson efficacy of 70% means that if, in an unvaccinated group of 100 persons, 20 develop COVID, then in a Johnson 6 Johnson-vaccinated group of 100 persons, only 6 will develop COVID.

Vaccine Efficacy: Individual and Social

But the benefit of the vaccine is more than lowering one’s personal chances of infection to only 5% of what they would otherwise be from community exposure. For, additionally, the community exposure itself lowers when a significant portion of the community gets vaccinated. Thus, the total protection for the individual multiplies.

Efficacy: Vaccine vs “Natural” Community Infection

The efficacy of vaccination in producing COVID immunity far exceeds that of “natural” community infection.

Data suggests that recovered but unvaccinated people are 2.34 times more likely to be reinfected with COVID than fully vaccinated people.1

Although COVID-19 hasn’t been around a sufficient time for a long-term study, researchers from Yale and the University of North Carolina have examined reinfection data from six other human-infecting corona viruses. Their study, published in The Lancet Microbe, said, “People who don’t get vaccinated against COVID-19 should expect to be reinfected with the corona virus every 16 to 17 months on average.”2 Jeffrey Townsend, the lead study author, emphasized, “Those who have been naturally infected should get vaccinated.”

These findings are not surprising. First, “natural” infection has an added variable, namely, the size of inoculum infecting each patient. This may contribute to different final degrees of immunity. On the other hand, vaccination has a specific uniform dose administered to all subjects. This is one less variable in the immunity generating process.

Second, over the course of time, immunity to the original virus fades and, additionally, new virus variants develop. At least, through the development of booster doses, vaccines can keep up with these changes.

COVID Presence in Children

Children’s COVID vaccination sometimes receives inadequate attention because the course of COVID in children seems less aggressive than in adults. However, this deficit of attention is unjustified.

First, “less aggressive” does not mean “harmless.” As of this writing, 16 Pennsylvania children have died from COVID since the epidemic began3.

Second, COVID among children is increasing. Among Pennsylvania children 5-18 years old, 9,214 cases have occurred during the first week of December 2021. Among children 4 years and younger, there have been 2,159 cases.3

Third, an increase or significant presence of disease in any segment of the population increases spread of the present variant and facilitates the development of new variants as the virus acquires more patients in which it can transform itself.

It is for reasons such as this that Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases, has said that getting children vaccinated is an important step toward achieving herd immunity.4

A consequence of this is the significance of the COVID vaccine’s efficacy in children.

Vaccine Efficacy in Children

Given the importance of vaccinating children, it is fortunate that the vaccines seem highly effective in them.

Pfizer recently tested adolescents ages 12-15 with the two standard 30-microgram doses of its vaccine needed for “full vaccination.” Among the 2,228 trial participants, there appeared 30 confirmed symptomatic cases in the unvaccinated group vs. 0 in the vaccinated group.5

In another Pfizer study, in a group of 2,000 children ages 5-11, 2/3 of the kids received the pediatric dose of 10 micrograms while the other 1/3 was left unvaccinated. While 16 children in the unvaccinated group developed symptomatic and lab-confirmed COVID-19, only 3 in the twice-as-large, vaccinated group contracted COVID-19.6

Finally, another Pfizer study enrolled 2,260 adolescents ages 12-15. In the unvaccinated group 18 developed COVID-19, while in the vaccinated group, there were 0 infections.7


The evidence is clear, vaccination works to reduce the illness burden in the community, as well as to reduce the probability of infection, illness, and death to the individual. Furthermore, it promises to shut down the epidemic sooner, thus precluding the virus from morphing into even more dangerous variants. Vaccination is, to borrow the words of Abraham Lincoln, “the last best hope of earth.” 8

Thomas Falasca, DO


1 Pogored. (2021, August 24). Common COVID-19 Vaccine Myths Explained. Retrieved September 13, 2021, from

2 Townsend, J. P., Hassler, H. B., Wang, Z., Miura, S., Singh, J., Kumar, S., . . . Dornburg, A. (2021). The durability of immunity against reinfection by SARS-CoV-2: A comparative evolutionary study. The Lancet Microbe,2(12). doi:10.1016/s2666-5247(21)00219-6

3 Woodall, C. (2021, December 13). COVID-19 in Pennsylvania: Why the state's leading doctors are imploring kids to get vaccinated. Retrieved December 13, 2021, from

4 Ellis, R. (2021, March 31). Pfizer: Vaccine Shown 100% Effective in Kids 12-15. Retrieved December 13, 2021, from

5 Crist, C. (2021, November 23). Pfizer COVID Vaccine Is 100% Effective in Adolescents: Study. Retrieved December 13, 2021, from

6 Goodman, B. (2021, October 22). Pfizer Vaccine for Kids 90% effective in Preventing COVID-19. Retrieved December 13, 2021, from

8 Lincoln, Abraham. Annual Message to Congress -- Concluding Remarks, Washington, D.C., December 1, 1862

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