Ninety years after the deadly 1918 flu pandemic, the immune systems of survivors still remembered and could neutralize the murderous strain.
Will our body’s memory of COVID-19 stay just as sharp?
“Most people still have most parts of an immunological memory at eight months… and would be protected against a serious reinfection for several years in the future,” said virologist Shane Crotty of the Vaccine Discovery Division at La Jolla Institute for Immunology.
“But we’re not saying it will be everybody,” he added. “There are some people who really don’t make immunological memory.”
Like any memory, it’s hard to look into the future and predict what will be recalled or forgotten, experts agreed. We can only measure what’s right in front of us. And the memory created by a vaccine may be different from one made by infection.
So far – based on new studies that analyze six months to a year of data – this is what we know:
• Of people who have been infected, protective antibodies can still be detected up to 11 months later. Levels plummet within four months after infection, as expected, according to research at Washington University in St. Louis, Missouri, published in the journal Nature on Monday. But then they plateau. They don’t vanish.
• Of people who have been vaccinated, antibodies are detected six months later, with only minimal decline. This doesn’t mean they vanish after six months; that’s just all the data we have. After six months, Pfizer reported 91% efficacy; Moderna, 94%. While the Johnson & Johnson vaccine hasn’t been around long enough to have long-term data, antibodies were still high 70 days post-vaccination.
“I actually am very hopeful that we’re going to have long lasting durable immunity after vaccination,” said UC San Francisco infectious disease expert Dr. Monica Gandhi.
• People’s immune memories of COVID-19 vary, with different patterns seen in different individuals, according to research by the La Jolla Institute for Immunology, published in the journal Science.
“The immune system is very diverse because the human population is very diverse,” said Daniela Weiskopf, assistant professor at the La Jolla Institute for Immunology.
• In a major year-long study of infected people treated at military treatment facilities, the antibody memory was briefer and more variable in people who were younger or who had less severe COVID-19. Some people, such as those who are immunocompromised, can’t make much of a memory at all.
• Even as antibody levels fall over time, other parts of our immune system stay vigorous and could help protect us. According to the La Jolla Institute study, three other parts of our immune system — memory B cells and two types of T cells — were measurable in more than 90% of people six months after infection. Memory B cells churn out new antibodies whenever needed. T cells patrol the body for infected cells, then kill them.
Memory B cells are particularly important, because of their role in producing antibodies. They stay vigorous for at least 12 months after infection, according to a study posted online this week at BioRxiv. That’s the part of the immune memory retained by the 1918 flu survivors.
But if our immunologic memory falters, and re-infection occurs, it’s unlikely to be deadly. Why? Reduced antibodies may miss every incoming virus, but they’ll still catch a lot, so infection is milder, said Crotty. And T cells will combat infection.
Beyond six months to a year, will we need booster shots?
“It’s possible that there’ll be an ‘in between’ answer,” with booster shots recommended for some people but not others, based on individual vulnerabilities, said Crotty.
Eventually, if immune memory fades, a booster shot may be needed for vaccinated Americans, according to Dr. Anthony S. Fauci, the nation’s top infectious-disease expert.
But we may not need it for quite a while, he said. The timing of those jabs is unknown.
“I don’t anticipate that the durability of the vaccine protection is going to be infinite — it’s just not,” he told a Senate Appropriations subcommittee hearing on Wednesday. “So I imagine we will need, at some time, a booster. What we’re figuring out right now is what that interval is going to be.”
There’s a second reason for boosters: variants.
“Mutations of the virus could lead to evasion of vaccine-induced immunity,” said Dr. Walter Orenstein of the Emory University of School of Medicine’s Center of Excellence for Influenza Research and Surveillance.
“We may need to re-vaccinate not only with same vaccine,” he predicted, “but modify the vaccine so it will work.”
Moderna CEO Stéphane Bancel told its investors in April that a booster shot, especially for front-line workers, might be needed by the end of the year. Pfizer CEO Albert Bourla predicted that a booster will be needed within eight to 12 months of initial vaccination. But skeptics note that companies have a financial motivation to promote a booster dose earlier than is warranted.
Those who have been infected but not vaccinated can’t count on long-term protection, say experts. In general, vaccines tend to trigger a far more vigorous immune defense than a natural infection.
“Everybody needs to get vaccinated because we have no way to say who will respond and who will retain the memory and who will not,” said Weiskopf.
A key question remains unresolved: What level of immunity is needed to fully to protect people?
A jump in the number of “breakthrough infections” — when people get infected after a vaccine or prior infection — would tell us that immunity is waning or that a dangerous variant has arrived. Scientists are also doing lab work that links fading immunity and illness.
Here’s the good news: If we lose our protection, it won’t be all at the same time, say experts. Because of the long vaccine rollout, and variation in human immunity, there won’t be a surprise wave of sudden severe disease and death.
But we need to be watching, waiting and ready to get boosters, they said.
“We need ongoing surveillance,” said Orenstein.
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Author: Lisa Krieger