How effective are vaccines against Omicron?
December 20, 2021
The pandemic has brought many tricky epidemiological terms and ideas into everyone’s life. Two particularly complicated concepts are the effectiveness and effectiveness of vaccines. These are not the same. And over time, when new variants such as omicron appear, these also change. Melissa Hawkins is an epidemiologist and public health researcher at American University. She explains how researchers calculate how well a vaccine prevents disease, what affects those numbers, and how Omicron changes things.
1. What do vaccines do?
A vaccine activates the immune system to produce antibodies that remain in your body to fight future exposure to a virus. All three vaccines currently approved in the US – Pfizer-BioNTech, Moderna, and Johnson and Johnson vaccines – have shown impressive results in clinical trials.
2. What is the difference between the effectiveness and effectiveness of the vaccine?
All new vaccines must undergo clinical trials, where researchers test the vaccines on thousands of people to see how well they work and whether they are safe.
Effectiveness is a measure of how well a vaccine works in clinical trials. The researchers plan the studies to encompass two groups of people: those who will receive the vaccine and those who will receive a placebo. They calculate the effectiveness of the vaccine by comparing how many cases of the disease occurred in each group, vaccinated versus placebo.
Effectiveness, on the other hand, describes how well a vaccine works in the real world. It is calculated in the same way by comparing the disease between vaccinated and unvaccinated people.
Effectiveness and effectiveness are usually close, but not necessarily the same. How the vaccines work will differ slightly from the study results once millions of people are vaccinated.
Many factors affect a vaccine’s performance in the real world. New flavors like Delta and Omicron can change things. The number and ages of people participating in the studies are important. The health of the vaccinated is also important.
Vaccine uptake – the proportion of a population that is vaccinated – can also affect the effectiveness of the vaccine. When a sufficiently large portion of the population is vaccinated, herd immunity begins to come into play. Vaccines with moderate or even low effectiveness can work very well at the population level. Similarly, vaccines with high efficacy in clinical trials, such as coronavirus vaccines, may have lower efficacy and low impact if there is no high vaccine intake in the population.
The distinction between efficacy and efficacy is important because one describes the risk reduction achieved by the vaccines under experimental conditions and the other describes how this may vary in populations with different exposures and transmission levels. Researchers can calculate both, but they can’t design a study that measures both at the same time.
3. How do you calculate effectiveness and effectiveness?
Both Pfizer and Moderna reported that their vaccines showed greater than 90% effectiveness in preventing symptomatic Covid-19 infection. In other words, those who received the vaccine in the clinical trials were at 90% less risk of developing Covid-19 compared to those who did not receive the vaccine.
Imagine doing a vaccine study. They randomize 1,000 people to receive the vaccine in one group. They randomize another 1,000 to be given a placebo in the other group. Say 2.5% of the people in the vaccinated group get Covid-19 compared to 50% in the unvaccinated group. That means the vaccine is 95% effective. We determine this because (50% – 2.5%) / 50% = 0.95. So 95% indicate the reduction in the proportion of disease in the vaccinated group. However, a vaccine that is 95% effective does not mean that 5% of those vaccinated will get Covid-19. The even better news: Your risk of illness is reduced by 95%.
The effectiveness of the vaccine is calculated in the same way, but it is determined by observational studies. Early on, vaccines were well over 90% effective in preventing serious illness in the real world. But viruses change naturally, and this can change its effectiveness. For example, one study found that in August 2021, when the delta rose, 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 the new strain all decreased effectiveness in this case.
4. What about the Omicron variant?
Preliminary data on Omicron and vaccines is coming fast, showing the vaccine is less effective. Best estimates suggest that vaccines are about 30 to 40% effective at preventing infections and 70% effective at preventing serious illnesses.
A preprint study conducted in Germany – one that has not yet been officially verified by other scientists – showed that antibodies in the blood of people who had been completely vaccinated with Moderna and Pfizer were less effective in neutralizing the Omicron variant. Other small preprint studies in South Africa and England showed a significant decrease in antibody targeting to 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 strengthen immunity to Omicron?
Initial data show that a third dose would help boost the immune response and protection against Omicron, with estimates of 70% to 75% effectiveness.
Pfizer has reported that people who received two doses of its vaccine are susceptible to infection from Omicron, but that a third vaccination improves antibody activity against the virus. This was based on laboratory experiments with the blood of people who received the vaccine.
Booster doses can increase the level of antibodies and the ability of a person’s immune system to protect itself against Omicron. However, unlike the US, much of the world does not have access to booster doses.
6. What does it all mean?
Despite the lower effectiveness of vaccines against Omicron, it is clear that vaccines work and are among the greatest public health achievements. Vaccines have varying degrees of effectiveness and are still useful. The flu vaccine is typically 40-60% effective and prevents disease in millions of people and hospitalizations in more than 100,000 people annually in the United States.
After all, 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 protects society as a whole.
This article was republished by The Conversation under a Creative Commons license. Read the original article.
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