The pandemic has brought many misleading terms and ideas into everyone’s life.
Two particularly complicated concepts they are the efficacy and effectiveness of vaccines. They are not the same.
And as time passes and new variants such as omicron appear, one and the other also change.
Melissa Hawkins is an epidemiologist and public health researcher at American University in Washington.
She explains how researchers calculate how much vaccines prevent disease, what influences effectiveness and efficacy rates, and how omicron is changing things.
What do vaccines do?
A vaccine activates the immune system to produce antibodies that remain in the body to fight future exposure to a virus.
All three vaccines currently approved for use in the United States, those from Pfizer-BioNTech, Moderna, and Johnson & Johnson, all showed success in clinical trials.
What is the difference between the efficacy and the effectiveness of a vaccine?
All new vaccines must undergo clinical trials in which researchers test vaccines in thousands of people to see if they work and are safe.
Efficacy is the measure of how well a vaccine works in clinical trials. The researchers design the trials to include two groups of people: those who receive the vaccine and those who receive a placebo. They calculate the efficacy of the vaccine by comparing how many cases of the disease occur in each group, vaccinated versus placebo.
Effectiveness, on the other hand, describes how well a vaccine performs in the real world. It is calculated in the same way, comparing diseases between vaccinated and unvaccinated people.
Efficacy and effectiveness are usually close to each other, but they are not necessarily the same. How vaccines work will vary somewhat from trial results once millions of people are vaccinated.
Many factors influence the performance of a vaccine in the real world. New variants like delta and omicron can change things.
The number and age of people enrolled in trials are important. And the health of the people who receive the vaccine as well.
The proportion of the population that is inoculated can also influence its effectiveness.
Vaccines with moderate and even low efficacy can work very well at the population level.
Similarly, vaccines with high efficacy in clinical trials, such as coronavirus vaccines, may have less effectiveness and little impact if there is not a high acceptance of the vaccine in the population.
The distinction between efficacy and effectiveness is important: one describes the risk reduction achieved by vaccines in clinical trials, and the other describes how this can vary in populations with different exposure and levels of transmission.
Researchers can calculate both, but cannot design a study that measures both simultaneously.
How is efficacy and effectiveness calculated?
Both Pfizer and Moderna reported that their vaccines demonstrated greater than 90% efficacy in preventing symptomatic COVID-19 infection.
In other words, among individuals who received the vaccine in clinical trials, the risk of contracting COVID-19 was reduced by 90% compared to those who did not receive the vaccine.
Imagine how a vaccine trial is conducted: 1,000 people are randomly selected to receive the vaccine in a group. And another thousand people to receive a placebo in another group.
2.5% of people in the vaccinated group contracted COVID-19 compared to 50% in the unvaccinated group. This means that the vaccine is 95% effective.
This is determined as follows: (50% – 2.5%) / 50% = 0.95. Therefore, that 95% refers to how much the incidence of the disease is reduced in the group of those vaccinated.
However, just because a vaccine is 95% effective does not mean that 5% of vaccinated people will contract COVID-19. It’s even better news: Your risk of getting sick is lowered by 95%.
The efficacy of the vaccine is calculated in exactly the same way, but it is determined by observational studies.
In the beginning, vaccines were more than 90% effective in preventing serious diseases in the real world. But, by their very nature, viruses change and this can alter effectiveness.
For example, one study found that by August 2021, when the delta variant was on the rise, the Pfizer vaccine was 53% effective in preventing the most serious covid among nursing home residents who had been vaccinated in early 2021.
In this case, age, health problems, waning immunity, and the new variant were factors that reduced effectiveness.
What about the omicron variant?
Preliminary data on omicron and vaccines is coming in quickly and reveals less effectiveness of the vaccine.
Best estimates suggest that vaccinated are 30-40% effective in preventing infection and 70% effective in preventing serious disease.
A study carried out in Germany which has not been formally peer reviewed found that antibodies in blood collected from people fully vaccinated with Moderna and Pfizer showed reduced efficiency to neutralize the omicron variant.
Other studies carried out in South Africa and England, which also do not and are awaiting formal peer review, showed a significant decrease in the efficacy of the antibodies against the omicron variant.
More disruptive infections are expected, with a lower ability of the immune system to recognize omicron compared to other variants.
Do boosters increase omicron immunity?
Initial data establish that a third dose would help stimulate the immune response and protection against omicron, with estimates of effectiveness of between 70 and 75%.
Pfizer has reported that people who have received two doses of its vaccine are susceptible to omicron infection, but that a third injection improves the activity of antibodies against the virus.
This was based on laboratory experiments that used blood from people who received the vaccine.
Booster doses can increase the amount of antibodies and the ability of a person’s immune system to protect against omicron. However, much of the world does not have access to booster doses.
What does this all mean?
Despite the lower efficacy of omicron vaccines, it is clear that the vaccines work and are among the greatest public health achievements.
Vaccines have varying levels of efficacy and are still useful.
The flu vaccine is generally 40-60% effective and prevents illness in millions of people and hospitalizations in more than 100,000 people in the US annually.
They protect not only those who are vaccinated, but also those who cannot be vaccinated.
Vaccinated people are less likely to transmit COVID-19, reducing new infections and offering protection to society at large.
* Melissa Hawkins is Professor of Public Health at American University.
This note originally appeared on The Conversation and is published here under a Creative Commons license. You can read the original article here.
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