Updated 7th July 2022

COVID: How to get your immune system into fighting shape

Written byHannah Weber
Reviewed byShirin Khalili, PhD

As we enter a third winter battling COVID-19, it is essential to build and maintain a strong immune system. The immune system is responsible for fighting off pathogens, including bacteria and viruses, to keep you healthy.

Your gut health and immune system are linked in a variety of ways. While your immune system helps keep harmful invaders out of the gut, what you feed your gut microbiome also has an impact on your immunity.

ZOE runs the COVID-19 Symptom Study, which is the largest study in the world on the effects of COVID-19, as well as the Predict Program, which is the largest nutrition study in the world looking at links between diet and COVID-19. 

Findings from our research show exactly what actionable things you can do to strengthen your immune system — like eating a healthy, gut-friendly diet that is full of plants — and to help protect yourself against coronavirus and other harmful invaders.

In this article, we will take a look at what you can do to ensure your body is ready to fight the virus. You can also take a quiz to learn how you can eat the right foods tailored to your body’s unique responses, in order to be in the best health this season.

How does the immune system work?

Your immune system defends your body against pathogens that threaten your health, like harmful bacteria and viruses.

The immune system is made up of a variety of molecules, cells, organs, and tissues. They work together to detect and neutralize invading pathogens, like the coronavirus that causes COVID-19. 

When your immune system makes contact with a pathogen for the first time, it can recognize that the pathogen is not part of your body and can spring into action to defend you. 

One part of your immune system, called the innate immune system, provides a very rapid response to all pathogens. The cells that are part of the innate immune system surround the invader and swallow them or release molecules that attack and destroy it.

The other part, called the adaptive immune system, takes over if the innate immune system can’t get rid of the pathogen. It needs a little more time because it adapts its response specifically to each invader by making antibodies. 

Specialized cells have to learn how to make tailored antibodies against a particular pathogen within a few days.

Antibodies wane over time, but the memory of the original design remains. In the event of a future infection, identical copies can be mass produced. 

This means that your body can fight off the same pathogen if you come into contact with it again. 

Gut health and the immune system

You may not think that your gut has much to do with fighting off respiratory illnesses, but it actually plays a vital role in strengthening the immune system. 

Your gut is home to trillions of bacteria. These bacteria have what is called a symbiotic relationship with their “host” — that’s you.

You provide them with an ideal habitat and plenty of nutrients and, in turn, they support your metabolism and help train and shape the immune system

The balance of different types of bacteria in your gut can influence everything from digestion to depression.

In a unique study, researchers at ZOE identified 15 "good" microbes associated with good gut health and 15 "bad" gut microbes associated with poor gut health. Some of these microbes are specifically associated with inflammation, a type of immune response.

The gut microbiome is intrinsically linked to the immune system, so if the body experiences things that strip away healthy bacteria (such as a poor diet, antibiotics, or chemotherapy), it can lead to reduced or inappropriate immune responses.

On the other hand, the immune system is essential for maintaining a good balance of bacteria in the gut.

Think about the last time you had a stomach bug — in that case, the immune system was unable to fight off a pathogen like norovirus before it caused unpleasant digestive symptoms.

The immune system is highly complex. It isn’t just one part of the body that you can focus on “boosting,” like doing squats for leg strength. It requires careful attention to many parts, including the gut. Luckily, getting into good habits can support the immune system over time.


The only way to boost your immune system specifically against COVID-19 is to introduce it to the invaders it needs to fight. You can do this by getting vaccinated.

There are different types of vaccines that work in slightly different ways, but they all aim to train the immune system to fight off COVID-19 without having to get the illness first. 

This is why some people experience mild flu-like symptoms, like body aches and chills, after getting vaccinated — their immune system is working hard to learn to fight the threat.

Vaccines are vital to immunity because they aren’t a “real” threat. They contain a harmless part of the virus that is specifically designed to cause a strong immune response.

The first dose introduces your body to the virus so it recognizes it in the future and can build up the right response, while the second dose further strengthens this specific immune response.

The ZOE COVID Symptom Study and countless other research studies have found that getting two or more doses of a COVID-19 vaccine reduces both your chances of getting the virus and your risk of getting seriously ill if you do catch it.

Our study, which is the largest of its kind anywhere in the world, also found that vaccination cuts the risk of developing long COVID in half.

Lifestyle changes to improve the immune system

There’s no magic pill that will boost your immune system, but there are modifiable lifestyle changes and actions you can take to strengthen your immunity. 

Diet and gut health

We already know that your diet affects your gut health, and that gut health is linked to immunity. 

A ZOE study, which included nearly 600,000 contributors, found that the risk of severe COVID-19 requiring hospital treatment was 40% lower if people ate a healthy diet, rich in plants and gut-friendly fermented foods, versus having an unhealthy diet. 

A healthy diet includes:

A 2021 study of over 2,800 people in six countries also found that people with plant-based or pescatarian (no meat except for fish) diets were less likely to have moderate to severe COVID-19. 

Of course, diet alone does not necessarily mean a person will have a strong immune system that can fight off COVID-19 and other illnesses.

People with high quality diets may also be exercising more, getting better quality sleep, or live in places with access to vital preventative healthcare.

You can take our free quiz to find out more about how ZOE can help you eat the foods that are best for your body, based on your unique responses to food and your gut microbiome.


People with obesity are more prone to infections than those who have a moderate weight.

The likelihood of having severe COVID-19 is higher in people with obesity, even if they are otherwise young and healthy.

A large review of 54 studies found that people with obesity had a 45% higher risk of developing COVID-19 than people without obesity and that they were more likely to die from the disease. 

The researchers suggested that an excessive immune response may make COVID-19 worse in those with obesity. 

Data from ZOE’s COVID Symptom Study also showed that people with overweight or obesity were more likely to experience severe symptoms of COVID. Other U.K. studies have found similar results.

Losing weight can be challenging, and it may help to get support from a doctor or nutritionist. 

If you want to understand more about your personal metabolic control, ZOE can help. Our at-home testing kit analyzes your blood sugar and blood fat responses, as well as your gut microbiome. Using the latest science, your personalized ZOE program helps you identify the best foods for your body. 


Sleep is very important for your immune system. Research has shown that chronic poor sleep changes how well the immune system functions and is linked with inflammation and worse health outcomes. 

There is also some evidence to show that not sleeping enough can reduce how well vaccines work.

Getting enough sleep in today’s busy world can be challenging. But prioritizing your bedtime is important for your immune system and your overall health. 

Other lifestyle changes

Other lifestyle changes that can impact the health of your immune system include:

  • getting enough exercise

  • reducing stress (such as by managing your workload and taking regular breaks)

  • quitting smoking

  • avoiding alcohol

Do supplements work?

There are no supplements proven to help fight COVID-19 or reduce your chances of getting it. 

Manufacturers that claim their supplements fight COVID are likely just looking to make money. This isn’t to say that their supplements are harmful, but they are not a replacement for getting vaccinated and taking other proven precautionary measures, like wearing a mask.

Vitamin D

Some research indicates that taking vitamin D can help protect against respiratory illnesses, like the common cold or the flu. 

Large-scale clinical trials are underway to figure out whether vitamin D can help protect against COVID-19. But right now, there is not enough good quality evidence to answer this question. 

However, many people’s vitamin D levels are lower than recommended. Speak to a healthcare professional about your vitamin D levels and what you can do to make sure you have enough of this vital vitamin. 

Vitamin C

Vitamin C is a popular supplement for preventing colds and flus, but there is little evidence to say that it actually works.

In fact, research shows that while getting enough vitamin C from your diet is beneficial for immunity and may help prevent infections, there is no data to support taking supplements for the same effect.

Probiotics, omega-3s, and more

The ZOE COVID Symptom Study also found that women (but not men) taking probiotics, omega-3 fatty acids, multivitamins, or vitamin D supplements had a lower risk of testing positive for COVID-19. 

Omega-3 fatty acids are important for both the innate and the adaptive immune system, while the role of other vitamins and probiotics is not as well-established. 

Since this was not a controlled study specifically on supplements, there is no way of knowing if the supplements themselves decreased the COVID risk. 

It could also simply mean that women who regularly take probiotics or other supplements are more likely to engage in other protective behaviors — for example, eating a nutritious diet, wearing masks, or limiting their exposure to large groups of people.

The best way to support the immune system to do its job is to eat a healthy, balanced diet with a wide variety of foods. These foods should provide all the nutrients, vitamins, and minerals you need without having to take supplements. 

If you’ve had a diagnostic test and know you have a nutritional deficiency, follow your doctor’s advice about supplements.


Having a healthy immune system doesn’t guarantee that you won’t get COVID-19. It is still vital to wear a mask, limit close contact with others, and wash your hands regularly. 

The best way to strengthen your immune system against coronavirus is to get fully vaccinated and take a booster shot when offered.

However, taking steps to boost your immune system can support your overall health and help prevent other common illnesses, like the cold or flu. 

The gut and immune system are mutually supportive — your immune system helps keep your gut healthy, and what you eat and drink can have a significant impact on your immunity.

Take a quiz to find out how ZOE can help you eat the right foods for your body.


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