Being vaccinated against rubella, or experiencing the rubella virus does not guarantee you are immune to the virus. A person's immunity can diminish over time. Therefore, while you may have been immune as a teenager, this may have become ineffective by the time you start having children. Similarly, you may have been immune for an earlier pregnancy, but not immune for a subsequent pregnancy. Occasionally, a person's body does not respond to the infection or vaccination, and they remain non-immune, even after experiencing a Rubella infection or being vaccinated.
It is only possible to know if you are immune to rubella by having a blood test. This test is called a 'rubella titre' and may be performed by your doctor if you are planning to have a baby. If you are not immune before you conceive, then your doctor may offer you a rubella vaccination. However, it is recommended that you should not be pregnant for at least 2 months after the vaccination is given. A further blood test is usually performed about 2 months after being vaccinated. This blood test can confirm whether you are immune or not. Occasionally, a person's body does not respond to the vaccine (in about 5% of people), and they remain non-immune, even after the vaccination. In this case, usually a second vaccination is recommended. If the second vaccination also fails to produce immunity, it is presumed that an immune response cannot be stimulated in that person.
NOTE: It is a general recommendation that women should not be pregnant for at least 1 to 2 months after having a rubella vaccination. However, a surveillance of 400 women in the UK, USA and Germany, who had inadvertently received the rubella vaccine before they knew they were pregnant, continued their pregnancies. None of these babies were affected.
A rubella titre blood test is also routinely performed on pregnant women at their first pregnancy visit.