Melissa Hawkins, American University
The pandemic has brought many tricky terms and ideas of epidemiology into everyone’s lives. Two particularly complicated concepts are the efficacy and efficacy of vaccines. They are not the same things. And as time goes by and new variants like omicron emerge, they change too. Melissa Hawkins is an epidemiologist and public health researcher at American University. She explains how researchers calculate how well a vaccine prevents disease, what influences those numbers, and how omicron is making a difference.
1. What are vaccines used for?
A vaccine activates the immune system to make antibodies that remain in your body to fight against exposure to a virus in the future. The three vaccines currently approved for use in the United States – the Pfizer-BioNTech, Moderna, and Johnson & Johnson vaccines – have shown impressive success in clinical trials.
2. What is the difference between the effectiveness and the effectiveness of a vaccine?
All new vaccines must undergo clinical trials in which researchers test vaccines on thousands of people to examine their effectiveness and safety.
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 effectiveness of the vaccine by comparing the number of cases of disease in each group, vaccinated versus placebo.
Efficacy, on the other hand, describes the performance of a vaccine in the real world. It is calculated in the same way, comparing disease in vaccinated and unvaccinated people.
Efficiency and effectiveness are generally close to each other but will not necessarily be the same. How vaccines work will vary somewhat from test results after millions of people have been vaccinated.
Many factors influence the performance of a vaccine in the real world. New variants like delta and omicron can make a difference. The number and age of people registered in the trials are important. And the health of those who receive the vaccine is also important.
The uptake of the vaccine – the proportion of a population that gets vaccinated – can also influence the effectiveness of the vaccine. When a large enough proportion of the population is vaccinated, herd immunity begins to kick in. Vaccines of moderate or even low efficacy can work very well at the population level. Likewise, vaccines with high efficacy in clinical trials, such as coronavirus vaccines, may have lower efficacy and low impact if there is not a high uptake of vaccines in the population.
The distinction between efficacy and efficacy is important, as one describes the risk reduction achieved by vaccines under test conditions and the other describes how this may vary in populations with different levels of exposure and transmission. Researchers can calculate both, but they can’t design a study that will measure both simultaneously.
3. How do you calculate efficiency and effectiveness?
Pfizer and Moderna have both reported that their vaccines have been shown to be over 90% effective in preventing symptomatic COVID-19 infection. In other words, among people 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 running a vaccine trial. You randomize 1,000 people to get the vaccine into a group. You randomize another 1,000 to receive a placebo in the other group. Suppose 2.5% of people in the vaccinated group receive COVID-19, compared to 50% in the unvaccinated group. This means that the vaccine is 95% effective. We determine this because (50% – 2.5%) / 50% = 0.95. Thus 95% indicates the reduction in the proportion of the disease among the vaccinated group. However, a vaccine with 95% effectiveness does not mean that 5% of people vaccinated will get COVID-19. It’s even better: your risk of disease is reduced by 95%.
The effectiveness of the vaccine is calculated in exactly the same way, but it is determined by observational studies. At first, vaccines were over 90% effective in preventing serious illness in the real world. But, by their very nature, viruses change, and this can change their effectiveness. For example, one study found that in August 2021, when the delta increased, the Pfizer vaccine was 53% effective in preventing serious illness in nursing home residents who had been vaccinated in early 2021. Age, health problems, declining immunity and new strain all reduced effectiveness in this case.
4. What about the omicron variant?
Preliminary data on omicron and vaccines is coming quickly and shows that the vaccines are less effective. Best estimates suggest that vaccines are around 30-40% effective in preventing infections and 70% effective in preventing serious illness.
A preprint study – which has not yet been officially reviewed by other scientists – which was conducted in Germany found that antibodies in the blood taken from people fully vaccinated with Moderna and Pfizer had reduced effectiveness in neutralizing the omicron variant. Other small preprint studies in South Africa and England showed a significant decrease in the ability of antibodies to target the omicron variant. More breakthrough infections are expected, with a decreased ability of the immune system to recognize omicron compared to other variants.
5. Do boosters boost immunity against omicron?
Initial data support that a third dose would help boost the immune response and protect against omicron, with efficacy estimates of 70-75%.
Pfizer has reported that people who have received two doses of its vaccine are susceptible to infection with omicron, but that a third injection improves antibody activity against the virus. This was based on laboratory experiments using the blood of people who have 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, unlike the United States, much of the world does not have access to booster doses.
6. What does all this mean?
Despite the reduced efficacy of omicron vaccines, it is clear that the vaccines work and are among the greatest achievements in public health. Vaccines have varying levels of effectiveness and are always useful. The flu vaccine is typically 40-60% effective and prevents illness in millions of people and hospitalizations in over 100,000 people in the United States each year.
Finally, vaccines protect not only those who are vaccinated, but also those who cannot be vaccinated. People who are vaccinated are less likely to spread COVID-19, which reduces new infections and provides protection for society as a whole.[Get The Conversation’s most important coronavirus headlines, weekly in a science newsletter]
Melissa Hawkins, Professor of Public Health, American University
This article is republished from The Conversation under a Creative Commons license. Read the original article.