What does a varicella zoster (VZV) positive test result mean?

What a Varicella Zoster Virus (VZV) Positive Test Result Means

A positive varicella zoster virus (VZV) test result indicates immunity to varicella (chickenpox) through either previous infection or vaccination, and the person is generally protected against developing chickenpox. 1

Understanding VZV Immunity

A positive VZV test typically detects the presence of VZV-specific antibodies in the blood, indicating one of the following:

• Previous natural infection: The person has had chickenpox in the past
• Successful vaccination: The person has developed immunity after receiving the varicella vaccine
• Recent or current infection: In some cases, a positive test could indicate an active or recent VZV infection

Types of VZV Testing

Different types of tests provide different information:

1. VZV IgG antibody test (most common):

   • Positive result indicates long-term immunity
   • 97-99% of adults with a history of chickenpox are seropositive 1
   • Even 71-93% of adults with negative or uncertain history are seropositive 1

2. VZV IgM antibody test:

   • Indicates recent or active infection
   • Only positive for approximately 3.5 weeks after onset of herpes zoster (shingles) 2
   • Less commonly used for routine immunity screening

Clinical Implications

For Healthcare Workers

A positive VZV test for healthcare workers has important implications:

• Permitted patient care: Healthcare personnel with evidence of immunity can care for patients with varicella or herpes zoster 1
• No work restrictions after exposure: Healthcare workers with documented immunity don't require furlough after exposure to VZV 1
• No need for post-exposure prophylaxis: Immune individuals don't require VZIG (varicella zoster immune globulin) after exposure 1

For General Population

For the general public, a positive VZV test means:

• Protection against chickenpox: The individual is likely protected against developing chickenpox
• Potential for herpes zoster: Despite immunity to chickenpox, the person may still develop herpes zoster (shingles) later in life as the virus remains dormant in nerve ganglia 3
• No need for vaccination: If immunity is confirmed, varicella vaccination is unnecessary

Important Considerations and Limitations

1. Test sensitivity varies:

   • Commercial assays may lack sensitivity to detect vaccine-induced immunity 1
   • False negatives can occur, especially in vaccinated individuals

2. Immunity verification:

   • A reliable history of chickenpox is considered valid evidence of immunity 1
   • For healthcare settings, serologic screening of personnel with negative or uncertain history is cost-effective 1

3. Special populations:

   • Immunocompromised patients may have positive antibody tests but still be susceptible to infection 4
   • In healthcare settings, a less sensitive but more specific ELISA test is recommended for screening 1

4. Breakthrough infections:

   • Despite positive antibody tests, breakthrough infections can occur, especially in vaccinated individuals
   • These are typically milder with fewer lesions and shorter duration 1

Clinical Decision-Making Algorithm

When interpreting a positive VZV test:

1. Consider test type:

   • IgG positive → Past infection or vaccination (long-term immunity)
   • IgM positive → Recent or current infection (within past 3.5 weeks)

2. Evaluate clinical context:

   • No symptoms + IgG positive → Immune, no further action needed
   • Symptoms present + IgG or IgM positive → Consider active infection, treat accordingly

3. For healthcare workers:

   • Positive VZV test → Can care for VZV-infected patients
   • Exposure to VZV after positive test → Monitor but no work restrictions needed

Remember that while a positive VZV test generally indicates immunity to chickenpox, it does not prevent the development of herpes zoster (shingles) later in life, which represents reactivation of the latent virus.