A panel of independent advisers from the Centers for Disease Control and Prevention will meet on Thursday to discuss whether some patients might need an extra dose of a Covid-19 vaccine.
There is growing concern that patients with weakened immune systems – including those with autoimmune diseases, HIV-infected patients, cancer patients and organ transplant recipients – may remain vulnerable to Covid-19 even after a full vaccination.
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These patients may need a second or third dose of a vaccine, depending on the vaccine they received. (Moderna and Pfizer-BioNTech’s mRNA vaccines require two doses, while the Johnson & Johnson vaccine requires only one.)
Israel, for example, has started offering third doses of the Pfizer vaccine to people with weakened immune systems.
Although vaccines always appear to reduce the risk of hospitalization and death in immunocompromised people, “there is clearly a gap between these patients and those with normal immune systems,” said occupational medicine expert Dr. Melanie Swift, Co-Chair. of the Mayo Clinic Covid-19 Vaccine Allocation and Distribution Working Group.
Dr Celine Gounder, infectious disease specialist and epidemiologist at New York University at Bellevue Hospital, said such an update to vaccine recommendations “wouldn’t mean vaccines don’t work.”
“This means that we are always looking for the best dosage regimen,” Gounder said.
As the highly contagious delta variant spreads across the country, “it’s all the more important that we have a very strong immune response” from the vaccines, Gounder said. The CDC reported on Tuesday that the variant accounted for 83% of new cases in the United States
It is not uncommon for vaccines to require multiple doses. The human papillomavirus vaccine is given in two doses, and a third is given to people with weakened immune systems. Two or three doses of the vaccine to prevent meningitis B are also needed, depending on the type of vaccine. And the hepatitis B vaccine is given in up to four injections.
The independent expert group, called the Advisory Committee on Immunization Practices, is not expected to suggest any changes to the vaccine guidelines at Thursday’s meeting. The panel also discussed additional doses last month, concluding that there was not yet enough evidence to suggest they were needed for the general population.
But strengthening immunity may be necessary in some groups. A study of cancer patients published in June found that while these patients responded well to vaccines, it was not yet clear how long their protection could last.
Further research conducted in May revealed variable immune responses in some cancer patients, “particularly patients with blood cancers, what we call hematologic malignancies, which also require highly immunosuppressive drugs,” Gounder said.
Organ transplant recipients may also need additional doses. They have to take medicine to suppress their immune system so that their body does not reject new organs. These drugs can also decrease the body’s response to vaccines.
A small study of 30 organ transplant recipients published in June suggested the need for a third dose. Although all participants had been fully vaccinated with Moderna or Pfizer-BioNTech vaccines, the majority were found to have no antibodies to the virus. Others had low levels of antibody response.
A third dose, the researchers said, boosted the antibody response somewhat, but more studies are needed to determine which patients could benefit the most.
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The question of whether an additional dose would ultimately be necessary can be analyzed in two ways. Do fully immune immunocompromised patients have breakthrough infections that make them sick? And do the blood samples from these patients indicate lower levels of immunity than those from patients with normal immune systems?
Examining the immune response goes beyond measuring antibody levels, Swift said.
“Some of these people will never develop a measurable neutralizing antibody, but does that mean they are not protected?” she said. Antibodies are not the only measure of the power and strength of the immune system. Other cells, such as T cells and memory B cells, also play a role, but they are not easily measured in laboratory tests.
The three manufacturers of Covid-19 vaccines licensed in the United States have investigated the safety and effectiveness of additional doses.
The National Institute of Allergy and Infectious Diseases has also started a clinical trial to determine whether a third injection of the Moderna vaccine could be given after a person has already been fully immunized with the Pfizer or Johnson & Johnson vaccines.
The results of these tests are expected this summer.
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