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Last week, Ark Valley Voice introduced a weekly COVID-19 Q and A column written by Dr. Lydia S. Segal in conjunction with Chaffee County Public Health. This column is focused on questions readers have about COVID-19 news and science.  As Dr. Segal points out, ‘Everything I write today is valid for today. COVID-19 news and science are rapidly evolving; assume updates will be made”.

This week’s column is about efficacy. What does the term mean, how is it calculated and what about that 95 percent that keeps getting reported?  When Pfizer and Moderna designed their vaccine clinical trials, they asked the question: ‘did you get COVID-19 after getting the vaccine?’

They did not ask these questions: did you get COVID-19 and not have symptoms, how severe were your symptoms (if any) and if you got sick, were you hospitalized?

QUESTION: What does efficacy mean and how is it calculated?

ANSWER: Efficacy is the relative difference in the percentage of people who get a disease who were unvaccinated in the placebo group compared to the percentage who got the disease who were vaccinated. In a summary version of the data, the difference in the infection risk of 0.74 and 0.04 = 0.70.  The efficacy is the relative difference of those two fractions  0.70/0.74 = 95 percent.

Group        Size       # Infected       Infection Risk  

Placebo      21,830       162                   0.74

Vaccine      21,830        8                      0.04

QUESTION: How are effectiveness and efficacy related?

ANSWER: Efficacy is a reflection of how the vaccine does in a clinical trial. Effectiveness is a reflection of how the vaccine does in the general population. Even though thousands signed up for the trial, it does not mean that the enrollees accurately reflect the population as a whole. The fact is, that if more people with different medical problems get the vaccine it might influence its effectiveness. Some segments of the population were excluded from the trial such as pregnant women and children.

QUESTION:  Does efficacy take into account those who were asymptomatic, i.e. without symptoms?

ANSWER: Remember that the vaccine trials were designed to look at whether people got symptoms of COVID-19 and if they did, they were tested. In general, it is estimated at least 25 to 40 percent of people who get COVID-19  are asymptomatic. There is no way to know how many people in the placebo or vaccinated group got COVID-19 but did not develop symptoms and therefore were not tested. It is postulated that asymptomatic carriers are less likely to transmit the virus, but that is yet to be fully tested. None of the asymptomatic are represented as positive in either the vaccinated or placebo groups.

QUESTION: How did they know the vaccinated people who got COVID had less severe cases?

ANSWER: Although the question of severity was not directly asked in the clinical trial, it was in fact observed that not one of those vaccinated who got symptomatic COVID-19 died. Also, the symptoms of those vaccinated who got COVID-19 were milder than in the general population. Therefore, the researchers concluded that even if one got COVID-19 after vaccination, their symptoms were likely to be milder, more like a cold or the flu.

QUESTION: Does the 95 percent efficacy mean I likely won’t get COVID-19?

ANSWER: At this point, the simple answer is yes. Keep in mind though, we still do not know if you can get such a mild case as to be COVID-19 positive but symptom-free. We also don’t know if asymptomatic vaccinated folks can pass the virus. This is the reason the CDC strongly suggests continued mask-wearing and social distancing.

QUESTION: The vaccines that are just coming out are reporting less efficacy. How come?

ANSWER: The answer has two parts. The first is the science behind how they are made is different, so they might in fact have different efficacy rates, meaning how effective they are at preventing disease. And one of the two, Johnson & Johnson is only one shot. If it was two shots, it might have higher efficacy.

The second part of the answer has to do with the variants (the strains from the United Kingdom, South Africa and Brazil) we are now seeing.  The Pfizer and Moderna vaccines were tested in the spring and summer before the variants were starting to spread, whereas the other vaccines started clinical trials later in the year when the variants started to become more prevalent. We don’t know what the efficacy would be for Pfizer and Moderna if they were in clinical trials at the same time.

By Lydia S. Segal, MD, MPH

Dr. Segal is a board-certified family physician who is attempting to retire. She holds a Masters in Public Health, and co-teaches with members of the Pelvic Physical Therapy staff at HRRMC classes on men’s and women’s pelvic health. In her former life, she was a general assignment reporter. In her spare time she hikes, skis and cooks.


Efficacy by Zimmer: summary chart with simple math, also used for teacher lessons