“We can’t tell you simply on the basis of the amount of antibody in your laboratory report how protected you are,” agreed William Schaffner, MD, professor of infectious diseases at Vanderbilt University, Nashville, Tenn., who is a spokesperson for the Infectious Diseases Society of America.
Not enough research has been done to establish that relationship, and results vary from one laboratory to another, he said.
That doesn’t mean that antibody tests don’t help, Dr. Schaffner said. On the basis of views of other experts he has consulted, Dr. Schaffner recommends that people who are immunocompromised undergo an antibody test. If the test is positive – meaning they have some antibodies to SARS-CoV-2, however low the titers – patients can take fewer precautions than before they were vaccinated.
But they should still be more cautious than people with healthy immune systems, he said. “Would I be going to large indoor gatherings without a mask? No. Would I be outside without a mask? Yes. Would I gather with three other people who are vaccinated to play a game of bridge? Yes. You have to titrate things a little and use some common sense,” he added.
If the results are negative, such patients may still be protected. Much research remains to be done on T-cell immunity, a second line of defense against the virus. And the current assays often produce false negative results. But to be on the safe side, people with this result should assume that their vaccine is not protecting them, Dr. Schaffner said.
That suggestion contradicts the Food and Drug Administration, which issued a recommendation on May 19 against using antibody tests to check the effectiveness of SARS-CoV-2 vaccination.
The studies so far suggest that vaccines are safe for people whose immune systems are compromised, Dr. Schaffner and Dr. Kamar agreed. Dr. Kamar is aware of only two case reports of transplant patients rejecting their transplants after vaccination. One of these was his own patient, and the rejection occurred after her second dose. She has not needed dialysis, although her kidney function was impaired.
But the FDA has not approved additional doses of SARS-CoV-2 vaccine to treat patients who are immunocompromised, and Dr. Kamar has not heard of any other national regulatory agencies that have.
In the United States, it may be difficult for anyone to obtain a third dose of vaccine outside of a clinical trial, Dr. Schaffner said, because vaccinators are likely to check databases and deny vaccination to anyone who has already received the recommended number.
Dr. Kamar, Dr. Mellors, and Dr. Schaffner have disclosed no relevant financial relationships.
A version of this article first appeared on Medscape.com.