Although two doses of a COVID-19 vaccine confer some protection for people who have received solid organ transplants, it's still not enough to enable them to dispense with masks, physical distancing and other safety measures, according to a study. This is a follow-up study to an earlier one in which the researchers reported that only 17 per cent of the participating transplant recipients produced sufficient antibodies after just one dose of a two-dose COVID-19 vaccine regimen.
"While there was an increase in those with detectable antibodies after the second shot, the number of transplant recipients in our second study whose antibody levels reached high enough levels to ward off infection was still well below than in people with healthy immune systems," said study lead author Brian Boyarsky from the Johns Hopkins University School of Medicine in the US.
"Based on our findings, we recommend that transplant recipients and other immunocompromised patients continue to practice strict COVID-19 safety precautions, even after vaccination," Boyarsky said.
The researchers noted that people who receive solid organ transplants, such as hearts, lungs and kidneys, often must take drugs to suppress their immune systems and prevent rejection.
Such regimens may interfere with a transplant recipient's ability to make antibodies to foreign substances, including the protective ones produced in response to vaccines, they said.
The latest study, published in the Journal of the American Medical Association (JAMA), evaluated this immunogenic response following the second dose of either of the two mRNA vaccines -- made by Moderna and Pfizer-BioNTech -- for 658 transplant recipients, none of whom had a prior diagnosis of COVID-19.
The participants completed their two-dose regimen between December 16, 2020, and March 13, 2021.
The researchers found that only 98 of the 658 study participants -- 15 per cent -- had detectable antibodies to SARS-CoV-2 at 21 days after the first vaccine dose.
This was comparable to the 17 per cent reported in the March study looking at the immune response after only one vaccine dose.
At 29 days following the second dose, the number of participants with detectable antibodies rose to 357 out of 658 -- 54 per cent, the researchers said.
After both vaccine doses were administered, 301 out of 658 participants -- 46 per cent -- had no detectable antibody at all while 259 -- 39 per cent -- only produced antibodies after the second shot, they said.
The researchers also found that among the participants, the most likely to develop an antibody response were younger, did not take immunosuppressive regimens including anti-metabolite drugs and received the Moderna vaccine.
These were similar to the associations seen in the March single-dose study, they said.
"Given these observations, transplant recipients should not assume that two vaccine doses guarantee sufficient immunity against SARS-CoV-2 any more than it did after just one dose," said study co-author Dorry Segev, from the Johns Hopkins University School of Medicine.