On 11 March 2020, coronavirus disease (COVID-19) was officially declared a pandemic by the World Health Organization.¹ The disease is caused by the SARS-CoV-2 virus.
In most people, the virus causes mild to moderate respiratory symptoms. However, particularly in older adults and those with pre-existing health conditions, COVID-19 can lead to serious illness and death.
As of March 2023, COVID-19 has claimed over 6.8 million lives.²
Early on in the fight against the virus, countries banded together to develop various medical solutions to beat the virus. These included a range of antiviral medications and vaccines, which continue to be the mainstay in preventing and treating COVID-19 infection.
However, a small but growing number of studies suggest that another possible way to fight COVID-19 may be probiotics — “good bacteria” that naturally exist in the body.
An example of such a study is a quadruple-blinded, placebo-controlled randomized trial (considered the gold standard of experimental design) conducted by scientists in Mexico. A total of 300 participants with COVID-19 took part in the study and were randomly assigned to either take a combination of four probiotics or a placebo.³
The researchers found that 53.1% of those in the probiotic group were asymptomatic and tested negative for the virus by day 30. Only 28.1% of the group taking the placebo were asymptomatic by day 20 and testing negative. The probiotics were also found to be well-tolerated by participants, meaning they caused no severe side effects.
Another group of researchers performed an analysis demonstrating that countries with high rates of fermented vegetable consumption had fewer COVID-19 deaths. They found that every 1g increase per day in the average amount of fermented vegetables consumed by a country was associated with a 35.4% decrease in the COVID-19 mortality rate.⁴
So, how exactly would something that mainly occurs in your digestive tract help with a respiratory infection? First, we need to understand probiotics.
You might assume you need to avoid all bacteria as they cause illness and infection. However, not all bacteria are bad — some types are actually good for you.
Collectively, all the good and bad microorganisms (including bacteria and viruses) that live in the human body are referred to as the microbiome.⁵ The helpful bacteria are known as probiotics. They can offer a range of health benefits for your gut, immunity, and more.⁶
Many people associate probiotics with yogurt, but they are also found in other fermented foods such as sauerkraut, kimchi, and miso. You can also get probiotics from supplements.
The most common probiotics used in products belong to the following families: Lactobacillus, Bifidobacterium, Saccharomyces, Streptococcus, Enterococcus, Escherichia, and Bacillus.⁷
One of the most well-known benefits of probiotics is their ability to improve gut health. They do this through several biological processes.
Probiotics can help strengthen the gut barrier — a layer of cells that lines the digestive tract, which helps keep out harmful bacteria and other toxins. Probiotics colonize the sites on the barrier so that toxins don’t adhere to it instead.⁸
They can also prevent the growth of bad bacteria by producing bacteria-killing substances like lactic acid, hydrogen peroxide, and bacteriocins.⁹
Some of these processes can also affect other parts of the body and even your overall immunity. In fact, probiotics can actually be found throughout the body.
For example, probiotics in your mouth help reduce plaque and prevent gum disease. They also play an important role in maintaining a healthy vaginal environment and preventing urinary tract infections.¹⁰
Probiotics can also have an indirect positive effect on other parts of the body through a cascade of biological processes.
For example, some probiotics can improve heart health by reducing cholesterol. They do this by either binding to cholesterol molecules or helping to make chemicals that help break down cholesterol.¹¹
Probiotics in the gut could have a similar connection to lung immunity, which experts have termed the “gut–lung axis (GLA)” or “gut–lung immune axis.” Experts suggest that this is also the most likely way in which probiotics help combat the SARS-CoV-2 virus.¹²
The SARS-CoV-2 virus enters the respiratory pathway when droplets of infected saliva enter another person’s body through the mouth or nose. Once inside, the virus starts attacking the cells along the airway and replicates there.¹³
So, what has all this got to do with the gut?
The stomach, intestines, and lungs are separate organs, but experts believe they are biologically linked through at a cellular level. This means that what happens in one area can affect another area.
This emerging concept is called the gut–lung axis.¹⁴
How these organs are linked is still unclear, but scientists have presented some general ideas. To understand the connection, it’s helpful to know how immunity works.
Your immune system is a network of cells, tissues, and organs that work together to defend your body.
When a virus or another invader enters your body, your immune system acts to try and fight it off. White blood cells are the main actors in your immune system. They come in several forms and work together to attack the invaders.
First, phagocytes engulf the invader and destroy it. Other white blood cells called B-cells produce antibodies, which attach to viruses to neutralize them. These B-cells can also activate T-cells to help them mount a larger attack.¹⁵
The gut–lung axis concept proposes that the immune system doesn’t act in isolation within each organ. This means changes in the intestinal microbiome can profoundly affect the immunity of other organs, such as the lungs. It’s proposed that the main connection between gut and lung immunity occurs through the mucosal tissue (soft tissue covering the organs and inner parts of the body), which essentially functions as one organ.
For example, during a COVID-19 infection, these immunity cells don’t just stay isolated in the lungs. When probiotics “activate” the immune cells in the gut, they can move to the lungs to trigger a stronger response to viral infection by producing more antibodies.¹⁶
This doesn’t just apply to COVID-19. Other researchers have also demonstrated a connection between the gut microbiome and other respiratory conditions like asthma, chronic obstructive pulmonary disease (COPD), and tuberculosis.
You can also find evidence for probiotics and lung health being closely intertwined in studies examining post-acute COVID-19 syndrome (PACS), or long COVID. In this condition, people continue to experience symptoms of COVID-19 for weeks or even months after the initial symptoms surfaced.
As a reference point, most people without other health complications recover within a few days or weeks. Post-COVID conditions can only be identified four weeks after the initial infection.¹⁷
While the exact number of people who have long COVID is unknown, studies suggest that around 5–50% of people who develop COVID-19 will go on to experience long-term symptoms. These can include fatigue, brain fog, and breathing difficulties.
While the exact cause of long COVID is still unknown, some studies suggest the gut microbiome may have something to do with it.
An analysis conducted by the Center for Gut Microbiota Research in Hong Kong found that those who experienced more severe symptoms of COVID-19 were also more likely to have an imbalance of good and bad bacteria. This is called “gut dysbiosis.”¹⁹
The researchers performed the analysis by collecting fecal samples from COVID-19 patients while they were hospitalized and compared them to healthy samples. In the COVID-19 group, researchers found more pathogens and fewer good bacteria in the samples. This imbalance even continued after the participants tested negative for COVID-19 and no longer had symptoms.¹⁸
The same team of researchers also conducted a study with people who had long COVID who were still experiencing symptoms six months after their initial infection. They found that patients without long COVID had a balanced microbiome that was similar to a healthy person, while those with long COVID were more likely to have gut dysbiosis.
Considered together, all this research is promising evidence that certain probiotic strains may help improve respiratory symptoms and shorten the duration of illness in COVID-19.
However, we must still interpret the results with caution.
Firstly, more studies are needed to validate what researchers have discovered. Furthermore, vaccination status wasn’t considered a potential influencing factor in the latest study in Mexico. This means we can’t be sure whether the effects were due to probiotics alone. The results may be skewed.
Other experts have suggested that studies should also look at other groups, including over 65s, who are most at risk of dying from the disease.
There’s still no concrete evidence that suggests you should consume specific foods to combat COVID-19. This is because the probiotics used in the studies mentioned above are not readily found in common foods.
So, what about probiotic supplements? Again, because the research in this area is still in its early stages, experts can’t recommend spending extra money on something that hasn’t been proven to work.
Those with severe illness or compromised immunity must also be careful about taking probiotics as they can cause harmful side effects in this population.²⁰
However, it’s always a good idea to maintain a balanced, fiber-rich diet to boost your overall immunity. Part of the reason for this is that probiotics need fiber to thrive. Good sources of fiber include fruits, vegetables, whole grains, legumes, and nuts.
One study involving 592,571 participants found that those who scored higher on a diet quality questionnaire (where they reported consuming more fruit and vegetables) had lower rates of COVID-19 infection and severity of symptoms.²¹
The Centers for Disease Control and Prevention (CDC) suggests the following ways to prevent COVID-19 infection and reduce the severity of symptoms:²²
Stay up to date with COVID-19 vaccinations to help lower your risk of death and severe symptoms. This also means getting booster shots when you are eligible to do so.
Stay in well-ventilated areas where possible. Poorly ventilated areas allow viral particles to build up in the air and increase your chances of infection. Try opening your windows to allow air to circulate or do activities outside if possible.
Avoid other people who are ill and may have COVID-19. If it’s not feasible for you to stay away from an infected person because you need to care for them, you can practice good personal hygiene to lower your risk of infection. This includes regular hand washing and maintaining distance.
Get tested for COVID-19 if you’re experiencing cold-like symptoms, including fever, sore throat, or cough.
If you are positive for COVID-19, follow the current isolation guidelines. If your symptoms are mild and can be managed at home, you can take over-the-counter medicines like acetaminophen or ibuprofen.²³
Seek treatment early if you’re at high risk of severe illness. There are now antiviral medications available to treat the symptoms of COVID-19, but they are only effective when taken within 5–7 days from when symptoms first appeared. This is why it’s important to seek medical treatment as soon as possible.
Wear face masks and socially distance yourself from others where possible.
Emerging evidence shows that probiotics could play a key role in preventing and treating COVID-19 in the future. However, research is still in its early stages. Experts are not recommending buying probiotic supplements just yet.
While eating probiotic-rich foods is always a healthy choice, it may not directly prevent you from getting COVID-19 or reduce its symptoms. This is because the probiotics used in current research aren’t those that are usually found in common foods like yogurt.
The best way to minimize your chances of getting COVID-19 and recovering well is by maintaining a balanced diet, staying up to date with vaccines, and practicing good personal hygiene.
WHO Director-General’s opening remarks at the media briefing on COVID-19 - 11 March 2020 | World Health Organization
WHO coronavirus (COVID-19) dashboard | World Health Organization
Probiotic improves symptomatic and viral clearance in Covid19 outpatients: a randomized, quadruple-blinded, placebo-controlled trial (2021)
Association between consumption of fermented vegetables and COVID-19 mortality at a country level in Europe (2020)
The microbiome — The nutrition source | Harvard T.H. Chan School of Public Health
Probiotics: What you need to know | NIH: National Center for Complementary and Integrative Health
Probiotics: Fact sheet for health professionals | NIH: Office of Dietary Supplements
Effects of probiotics on the improvement and regulation of intestinal barrier dysfunction and immune imbalance in intra‑abdominal infections (review) (2021)
Bacteriocin-producing probiotic lactic acid bacteria in controlling dysbiosis of the gut microbiota (2022)
Cholesterol-lowering effects of probiotics and prebiotics: A review of in vivo and in vitro findings (2010)
Basics of COVID-19 | Centers for Disease Control and Prevention (CDC)
Immune response | NIH: MedlinePlus
Probiotic improves symptomatic and viral clearance in Covid19 outpatients: a randomized, quadruple-blinded, placebo-controlled trial (2021)
Long COVID or Post-COVID conditions | Centers for Disease Control and Prevention (CDC)
Alterations in gut microbiota of patients with COVID-19 during time of hospitalization (2020)
Gut microbiota dynamics in a prospective cohort of patients with post-acute COVID-19 syndrome (2022)
Can probiotics help fight COVID-19? | National Academies
Diet quality and risk and severity of COVID-19: a prospective cohort study (2021)
How to protect yourself and others | Centers for Disease Control and Prevention (CDC)
COVID-19 treatments and medications | Centers for Disease Control and Prevention (CDC)
Dawn Teh is a health writer and former psychologist who enjoys exploring topics about the mind, body, and understanding what helps humans thrive.
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