How Effective Are COVID Vaccines Against the Omicron Variant? An Epidemiologist Explains

Understanding how much protection a vaccine provides is not as easy as it sounds.

The pandemic has brought many tricky terms and ideas from epidemiology into everyone’s life. Two particularly complicated concepts are the efficacy and effectiveness of vaccines. These are not the same. And as time goes by and new variants like omicron appear, 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 affects these numbers, and how omicron changes things.

1. What do vaccines do?

A vaccine activates the immune system to make antibodies that remain in your body to fight off exposure to a virus in the future. All three vaccines currently approved for use in the US – the Pfizer-BioNTech, Moderna and Johnson & Johnson vaccines – have shown impressive success in clinical trials.

Clinical trials are used to calculate the efficacy of a vaccine, but do not necessarily represent real-world conditions.

2. What is the difference between efficacy and effectiveness of vaccines?

All new vaccines must undergo clinical trials in which researchers test the vaccines on thousands of people to find out how well they work and whether they are safe.

Efficacy is the measure of how well a vaccine works in clinical trials. Researchers are designing the trials to include two groups of people: those who receive the vaccine and those who receive a placebo. They calculate vaccine efficacy 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 the same way, comparing disease among vaccinated and unvaccinated people.

Efficacy and effectiveness are usually close, but will not necessarily be the same. How the vaccines work will be a little different from the research results once millions of people are vaccinated.

Many factors influence how a vaccine performs in the real world. New variants such as delta and ommicron can change this. The number and age of the people who took part in the trials are important. And the health of those who receive the vaccine is also important.

Vaccine uptake – the proportion of a population that is vaccinated – can also affect the effectiveness of the vaccine. When a large enough proportion of the population is vaccinated, herd immunity starts to come into play. Vaccines with moderate or 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 lower effectiveness and small impact if there is not high vaccine uptake by the population.

The distinction between efficacy and effectiveness is important because one describes the risk reduction achieved by the vaccines under trial conditions and the other describes how this may vary in populations with different exposures and transmission levels. Researchers can calculate both, but they cannot design a study that measures both at the same time.

3. How do you calculate efficacy and effectiveness?

Both Pfizer and Moderna reported that their vaccines showed greater than 90% efficacy in preventing symptomatic COVID-19 infection. In other words, those who received the vaccine in the clinical trials had a 90% reduction in their risk of contracting COVID-19 compared to those who did not receive the vaccine.

Imagine you are doing a vaccine trial. You randomize 1,000 people to get the vaccine in one group. You randomize another 1,000 to get a placebo in the other group. Assume that 2.5% of people in the vaccinated group will get COVID-19, compared to 50% in the unvaccinated group. This means that the vaccine is 95% effective. We establish that because (50% – 2.5%)/50% = 0.95. So 95% indicates the reduction of the disease share among the vaccinated group. However, a vaccine with 95% efficacy does not mean that 5% of vaccinated people will get COVID-19. Even better news: Your risk of disease is reduced by 95%.

Vaccine effectiveness is calculated in exactly the same way, but is determined by observational studies. In the beginning, vaccines were more than 90% effective at preventing serious diseases in the real world. But by their very nature, viruses change, and this can alter their effectiveness. For example, one study found that by 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, waning immunity and the new strain all reduced effectiveness in this case.

New variants of the coronavirus are all slightly different from the original strain on which vaccines were based, so immunity to variants may be different. Credit: Alexey Solodovnikov, Valeria Arkhipova/WikimediaCommons, CC BY-SA

4. What about the ommicron variant?

Preliminary data on omicron and vaccines are coming in fast, revealing lower vaccine effectiveness. By best estimates, vaccines are about 30-40% effective at preventing infections and 70% effective at preventing serious illness.

A preprint study – one that has not yet been formally reviewed by other scientists – conducted in Germany found that antibodies in blood collected from people fully vaccinated with Moderna and Pfizer showed reduced efficacy in neutralizing the omicron. -variant. Other small preprint studies in South Africa and England showed a significant decrease in how well antibodies target the omicron variant. More breakthrough infections are expected, with a reduced ability of the immune system to recognize omicron compared to other variants.

Most people in the US are now eligible for a booster coronavirus vaccine that can help protect against the ommicron variant.

5. Do boosters increase immunity to ommicron?

Initial data confirm that a third dose would help boost immune response and protection against omicron, with estimates of 70-75% effectiveness.

Pfizer has reported that people who have received two doses of the vaccine are susceptible to infection by omicron, but that a third injection improves antibody activity against the virus. This was based on lab experiments with the blood of 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, unlike the US, much of the world does not have access to booster doses.

6. What does all this mean?

Despite the reduced effectiveness of vaccines against omicron, it is clear that vaccines work and are among the greatest public health achievements. Vaccines have varying levels of effectiveness and are still useful. The flu vaccine is usually 40%-60% effective and annually prevents illness in millions of people and hospitalizations in more than 100,000 people in the US.

Finally, vaccines protect not only those who have been vaccinated, but also those who cannot get vaccinated. Vaccinated people are less likely to spread COVID-19, which reduces new infections and protects society in general.

Written by Melissa Hawkins, professor of public health at the American University.

This article was first published in The Conversation.