Parasitic infections of the intestines are typically caused by two types of organisms:

  • Protozoa. These single-celled organisms can live and multiply inside your body. Infections caused by protozoa include giardiasis, an infection that you can get from drinking water infected with Giardia protozoa.
  • Helminths. These multi-celled organisms — commonly known as worms — can live in or outside of your body.

About 3 1/2 billion people worldwide are affected by intestinal parasites. People living in developing countries are disproportionally affected.

A recent study has found that intestinal parasites may lower the severity of coronavirus disease 19 (COVID-19). However, other studies have found that parasitic infections may prevent the immune system from responding effectively to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19.

For scientists, this topic of debate is complex and difficult to study. Much more research is needed to determine how intestinal parasites may affect a person’s immune reaction to a SARS-CoV-2 infection.

In this article, we review what we currently know about how intestinal parasites affect the immune system, SARS-CoV-2 infections, and COVID-19 vaccines. Keep reading to discover more.

When we talk about the effect of intestinal parasites on the human immune system, we’re often talking about helminths or parasitic worms.

Helminths are made up of multiple cells and are rather large. In fact, they can often be visible to the naked eye. Some examples of helminths that can infect humans include:

Potential benefits

Simply put, it’s been observed that helminths can dampen down specific parts of the immune response. Obviously, this is beneficial to the parasite, as it means the immune system is less likely to attack and kill it.

However, it may also have a beneficial effect on humans as well. This toned-down immune response may help to protect a person from things such as allergies, autoimmune conditions, and some inflammatory diseases.

In fact, a decrease in helminth infections has been proposed as a contributing factor to the rise of allergic and inflammatory diseases in developed countries. This is called the “hygiene hypothesis.”

Potential drawbacks

Despite the potential immune benefit from helminths, there are big drawbacks as well. Helminths can cause severe damage to the human host. As they grow in size, they can cause blockages in internal organs such as the intestines or bile duct.

A 2017 study also found that helminth infection may increase the risk of cancer.

In addition to causing severe disease themselves, helminths may make some infectious diseases more severe. And they may decrease the effectiveness of vaccines.

In severe COVID-19, high levels of inflammation can be present in the body. This is referred to as a “cytokine storm” and can cause serious complications such as acute respiratory distress syndrome (ARDS).

Could the dampening effects of intestinal parasites on the immune system help to protect against these high levels of inflammation?

While there’s some evidence that intestinal parasites may be associated with reduced COVID-19 severity, there’s also evidence that intestinal parasites can hinder the immune system’s ability to ward off a SARS-CoV-2 infection. Bottom line: more research still needs to be done.

Research showing parasites may reduce COVID-19 severity

A 2021 study was carried out in Ethiopia from July 2020 through March 2021. A total of 751 individuals with laboratory-confirmed COVID-19 were included in the study.

Out of 751 people, 284 (37 4/5 percent) also had a parasitic infection. When it came to COVID-19 severity, only 10 3/5 percent of people with severe COVID-19 had a parasitic infection. Additionally, none of these people died of COVID-19.

The researchers suggest that the higher prevalence of intestinal parasites in developing parts of the world, particularly in Africa, may contribute to the lower prevalence and severity of COVID-19 in these areas.

However, the study does have some limitations, most notably:

  • a small number of participants
  • potential bias

Conflicting views and research

A 2022 letter expresses skepticism about the findings above. In it, the writers express concerns about potential bias in the study, as participants were first tested for SARS-CoV-2 prior to being tested for intestinal parasites.

Additionally, a 2020 review notes that parasitic infections, particularly with helminths, have been previously found to increase the severity of other infectious conditions, such as HIV, tuberculosis, and malaria.

As such, the reviewer notes that it’s possible that having a parasitic infection could potentially inhibit the immune system from responding effectively to COVID-19. This may actually increase disease severity in some cases.

Vaccination is the most effective way to protect against serious illness and death due to COVID-19. However, in order to be effective, vaccines need to initiate an immune response. Could the immune effects of intestinal parasites affect this?

There’s actually some evidence that parasitic infections can decrease the effectiveness of vaccines. However, no research has assessed the effect that parasites have on COVID-19 vaccines specifically.

Research studies show parasites result in poorer vaccine outcomes

A 2020 review of studies looked to dive more deeply into the topic of parasites and vaccines. A total of 50 articles were analyzed. The researchers note that several parasite- and vaccine-related factors can affect vaccination, such as the:

  • specific type and stage of parasitic infection
  • timing of the infection, such as whether it’s acute or chronic
  • type of vaccine and how it’s given
  • kind of immune response that the vaccine aims to generate

After the analysis was completed, it was found that having a parasitic infection at the time of getting a vaccine results in a poorer vaccine outcome. Chronic helminth infections are more likely to negatively affect vaccination effectiveness.

Do parasitic infections impact the effectiveness of the COVID-19 vaccines?

No research has been done on how parasites may impact the effectiveness of the COVID-19 vaccines. This is certainly something that researchers will investigate in the future.

You may have a few more questions about intestinal parasites. Let’s answer some of them now.

How do you get intestinal parasites?

Many times, the eggs of an intestinal parasite are passed in a person’s stool. You can get an intestinal parasite if you ingest these eggs.

This can happen by coming into contact with fecal particles in the environment, particularly in the soil, and then touching your mouth without washing your hands first. Consuming contaminated food and water can also spread these parasites.

Some types of intestinal parasites, such as Strongyloides species, are transmitted through soil. In this case, contact with contaminated soil allows the larvae of this parasite to burrow into your skin and travel to your intestines.

What are the symptoms of an intestinal parasite infection?

The exact symptoms of an intestinal parasite infection can vary based on the specific parasite. Some people may not even have any significant symptoms. When symptoms are present, some to look out for include:

What happens if an intestinal parasite infection is left untreated?

If you suspect that you have an intestinal parasite infection, it’s important to see a doctor for diagnosis and treatment. When an intestinal parasite infection is left untreated, it can lead to complications such as:

Does an intestinal parasite coinfection affect treatment for COVID-19?

At the moment, the effect that a parasitic infection may have on COVID-19 treatments isn’t clear. Research into the interaction between intestinal parasites and COVID-19 is still in its early stages.

Intestinal parasites can dampen down certain aspects of the immune response in order to prevent being attacked by the immune system. This effect may protect against some diseases but may also worsen others.

There’s some evidence that intestinal parasites may reduce COVID-19 severity. However, this data comes from just one study and more research into this topic needs to be done.

If left untreated, intestinal parasite infections can cause complications. See a doctor if you have symptoms such as severe abdominal pain, diarrhea, or nausea and vomiting. They can help you find out what may be causing your symptoms.