Antiviral Resistance

What is antiviral resistance?

Antiviral resistance occurs when a virus stops responding to an antiviral medication. The virus changes, making an antiviral drug less effective or completely ineffective. A virus that develops antiviral resistance is harder to treat.

Antiviral resistance is a type of antimicrobial resistance. Antimicrobial resistance refers to viruses, bacteria, parasites and fungi that no longer respond to medications developed to treat them.

What are antiviral medications?

Antivirals treat certain contagious viruses that cause infections and illness, such as:

• Influenza (the flu).
• Genital herpes.
• Human immunodeficiency virus (HIV).

For certain viruses like HIV and herpes, antiviral drugs lower the chances of spreading the infection to others. You can also take antivirals to prevent getting sick after a suspected or known viral exposure.

How do antivirals work?

A virus causes infection by entering healthy host cells and making copies of itself (replicating). Antiviral medications block the virus’s ability to replicate.

One way antivirals fight off viruses is by preventing a virus from attaching to and entering cells. A virus without a host can’t multiply.

What causes antiviral resistance?

Antivirals help lower the amount of a virus (viral load), but the virus is always there. If you miss prescribed doses of an antiviral or stop treatment too soon, the virus may start multiplying again. As it replicates, the virus’s genetic makeup may mutate (change).

If a virus goes through too many changes, the antiviral drug won’t recognize the new virus variant (version of the virus). Once a virus is drug resistant, the medication can’t keep it from multiplying.

Sometimes, a virus stops responding to previously effective antivirals for no known reason. This effect is called spontaneous resistance.

Who is at risk for developing antiviral resistance?

People who take antivirals for long periods to treat chronic viral infections like HIV, genital herpes and hepatitis B or hepatitis C are more at risk for antiviral resistance.

People who have compromised immune systems due to autoimmune disease, organ transplants or cancer treatments like chemotherapy are also at greater risk for antiviral resistance.

Are antiviral-resistant viruses contagious?

Yes. This situation is called transmitted drug resistance. People who develop an antiviral-resistant strain of a virus can pass it to others through the exchange of bodily fluids like semen, saliva and blood. Pregnant people can also pass antiviral-resistant viruses to their babies during the birthing process.

With transmitted resistance, a virus is already resistant to specific antivirals even if you’ve never taken the drug. Your healthcare provider will try other antivirals.

How are antiviral-resistant viruses detected?

A blood test can check for antiviral resistance if you have HIV. There are two types:

• Genotypic antiretroviral resistance test (GART): Looks for drug-resistant changes in HIV genes.
• Phenotypic antiretroviral resistance test: Measures how the HIV strain responds to different concentrations of antiviral medicines.

The Centers for Disease Control and Prevention (CDC) also use blood tests to see if viruses like the flu and COVID-19 are becoming drug resistant.

How are antiviral-resistant viruses treated?

If a virus shows signs of developing antiviral resistance, your healthcare provider may increase the dose or switch you to a different antiviral drug. Some viruses have a limited number of approved antiviral treatments.

Some people with potentially life-threatening chronic viral infections like HIV may take two antiviral medicines at the same time. A virus is unlikely to develop resistance to both drugs at the same time. One drug is always working to keep the virus in check.

Can you prevent antiviral resistance?

The best way to prevent antiviral resistance is to take medicines exactly as prescribed. Set a reminder on your phone to ensure you take the medicine at the same time each day. Ask your healthcare provider what to do if you miss a dose. You may need to take the medicine as soon as you remember.

A note from Cleveland Clinic

Antiviral medications can keep potentially life-threatening viral infections in check. The drugs keep the virus from multiplying and causing symptoms. Failing to take an antiviral as prescribed can allow a virus to become active. The virus may change as it multiplies, making the antiviral ineffective against the new virus variant. If you develop antiviral resistance, your healthcare provider may change the medicine or the dose.