The varicella (chickenpox) vaccine protects individuals from the highly contagious varicella-zoster virus (VZV). It prompts the immune system to produce protective antibodies, establishing immunity. Confirming this immunity after vaccination is often necessary to ensure adequate protection.
What is a Varicella Titer?
A varicella titer is a blood test measuring immunoglobulin G (IgG) antibodies against the varicella-zoster virus. These antibodies indicate immunity to chickenpox, from past natural infection or vaccination. Sufficient antibody levels suggest the immune system can fight the virus upon exposure.
This test is commonly required for individuals such as healthcare workers, students, or international travelers to verify immune status. It provides documentation of protection, especially when vaccination records are unavailable or previous infection is uncertain. Titer results are interpreted as negative (no detectable antibodies), equivocal (antibodies present but not protective), or positive (protective levels).
When to Get a Varicella Titer
Timing a varicella titer after vaccination allows the immune system sufficient time to develop a protective antibody response. It is recommended to wait approximately 4 to 8 weeks after the final vaccine dose before testing. This waiting period ensures detectable IgG antibody production.
For children aged 12 months to 12 years, the varicella vaccine is typically administered in a two-dose series, with the second dose given at least three months after the first. Adolescents and adults aged 13 years or older who lack evidence of immunity also receive two doses, usually separated by a minimum of 4 to 8 weeks. After the completion of the two-dose series, most individuals (about 99% of adolescents and adults) develop detectable antibodies. While post-vaccination serologic testing is generally not recommended for everyone due to the high seroconversion rates after two doses, it may be performed in specific situations where proof of immunity is required.
Factors Affecting Titer Results
Several factors influence a person’s antibody response to the varicella vaccine and their titer results. Individual immune system differences mean not everyone responds identically. The number of vaccine doses also impacts antibody levels; two doses lead to higher, more sustained titers than a single dose.
The specific vaccine type or brand affects the immune response. Immunosuppression, whether due to medical conditions or medications, can weaken antibody production, potentially leading to lower or negative titer results. While a positive titer indicates protection, the exact numerical antibody level is not always the sole indicator of immunity, as some assays may not detect vaccine-induced antibody levels as effectively as those from natural infection.
Interpreting Low Titer Results
If a varicella titer result is low, negative, or equivocal after vaccination, it indicates insufficient antibody levels for confirmed protection. This doesn’t mean the vaccine was ineffective, but that the immune response wasn’t strong enough for the test. The next step is typically an additional varicella vaccine dose, often called a booster shot.
Following this additional dose, another titer test is recommended, typically 4 to 6 weeks later, to assess if a protective antibody level has been achieved. If the titer remains low after multiple doses, the individual might be considered a “non-responder.” It is advisable to consult a healthcare provider to interpret titer results and determine the most suitable course of action.