The Latest on COVID-19, Immunity, and Thrombosis Risk

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NEW ORLEANS — Addressing an audience of hematologists, an immunologist and a thrombosis specialist presented insights on two hot COVID-19 topics: strategies the virus uses to breach the immune system and the diagnosis and treatment of vaccine-related blood clots.

In a presidential symposium at the annual meeting of the American Society of Hematology, La Jolla Institute of Immunology scientist Shane Crotty, PhD, explained that COVID-19 has a “superpower” that allows it to be “extraordinarily stealthy.”

The virus, he said, can sneak past the body’s innate immune system, which normally responds to viral invaders within minutes to hours. “This is why you have people with high viral loads who are presymptomatic. Their innate immune system hasn’t even recognized that these people are infected.”

The adaptive immune system kicks in later. As Crotty noted, adaptive immunity is composed of three branches: B cells (the source of antibodies), CD4 “helper” T cells, and CD8 “killer” T cells. In the first year of COVID-19, his team tracked 188 subjects post infection in what he said was the largest study of its kind ever for any viral infection.

“In 8 months, 95% of people who had been infected still had measurable immune memory. In fact, most of them had multiple different compartments of immune memory still detectable, and it was likely that these individuals would still have that memory years into the future. Based on that, we made the prediction that most people who have had COVID-19 would likely be protected from reinfection — at least by severe infections — for 3 years into the future. That prediction has widely held up even in the presence of variants which weren’t around at the time.”

How do vaccines fit into the immunity picture? Crotty’s lab has tracked subjects who received 4 vaccines — Moderna, Pfizer/BioNTech, Janssen/Johnson & Johnson, and Novavax. Researchers found that the mRNA vaccines, Moderna and Pfizer/BioNTech, “are fantastic at eliciting neutralizing antibodies quickly, but then they drop off rapidly at two doses and actually continue to drop for 10 months.”

Still, he said, “when we take a look at 6 months, actually the vaccines are doing pretty incredibly well. If we compare them to an average infected individual, the mRNA vaccines all have higher neutralizing antibody titers.”

What’s happening? According to Crotty, B cells are “making guesses about what other variants might look like.” But he said research suggests that an important component of this process — germinal centers — aren’t made in some vaccinated people who are immunocompromised. (Germinal centers have been described as “microbial boot camps” for B cells.)

The good news, Crotty noted, is that a greater understanding of how COVID-19 penetrates various layers of adaptive immune defenses will lead to better ways to protect the immunocompromised. “If you think about immunity in this layered defense way, there are various ways that it could be enhanced for individuals in different categories,” he said.

Hematologist Beverley J. Hunt, MD, OBE, of St. Thomas’ Hospital/King’s Healthcare Partners in London, spoke at the ASH presidential symposium about blood clots and COVID-19. As she noted, concern arose about vaccine-related blood clots. A British team “managed quickly to come up with a diagnostic criteria,” she said. “We looked at nearly 300 patients and essentially came up with a scoring system.”

The diagnostic criteria was based on an analysis of definite or probable cases of vaccine-induced immune thrombocytopenia and thrombosis (VITT) — all related to the AstraZeneca vaccine. The criteria appeared in a 2021 study in the New England Journal of Medicine.

The report’s data didn’t allow it to compare the efficacy of anticoagulants. However, Hunt noted that clinicians turned to plasma exchange in patients with low platelet counts and extensive thrombosis. The report stated “survival after plasma exchange was 90%, considerably better than would be predicted given the baseline characteristics.”

“Now we’re following up,” Hunt said. One question to answer: Is long-term anticoagulation helpful? “We have many patients,” she said, “who are taking an anti-platelet factor out of habit.”

Crotty and Hunt report no disclosures. This reporter is a paid participant in a COVID vaccine study run by Crotty’s lab.

This article originally appeared on, part of the Medscape Professional Network.

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