Concerns about waning immunity from the Covid-19 vaccines have led to calls for booster shots. But a decrease in antibodies over time is "perfectly normal" and protection against severe disease and death remains strong even as protection against infection decreases, Katherine J. Wu writes for The Atlantic.

Your top resources on the Covid-19 vaccines

Explanations for Israel's vaccine effectiveness data

When Israeli researchers announced last month that people vaccinated in the winter were more likely to contract Covid than those who had been vaccinated in the spring, many assumed this demonstrated waning vaccine effectiveness.

Are you ready for booster shots? Start thinking about these 6 factors now.

The data from Israel's Ministry of Health suggested that vaccine effectiveness against both infection and symptomatic disease had decreased to around 40%, but this decline may have been due to other factors and not the vaccines, Wu writes. For example, when people were initially vaccinated, they were more likely to wear masks and social distance—but over time, they may have been less cautious, which could have increased their risk of infection.

And David Leonhardt, writing for the New York Times, also highlights that the first Israelis to receive the vaccine were more affluent, meaning they could have been more likely to travel and pursue other activities, also increasing their risk of exposure and infection.

But even as individuals' antibodies decline over time, Wu says immune responses don't last forever and are, in fact, meant to wane over time.

Is waning vaccine immunity problematic?

According to Wu, the first time a body encounters a vaccine or a pathogen, defensive cells from the innate immune system work to fight off the foreign matter, allowing time for antibodies, which are made by B cells, and killer T cells to form in the blood.

Initially, there are high amounts of these protective cells, but over time, as the threat of an infection decreases, the frontline B and T cells, which are no longer as needed, begin to die off. This causes antibody levels to decrease for several months before leveling off.

According to Deepta Bhattacharya, an immunologist at the University of Arizona, this decrease in antibodies over time is normal. "You have a big increase at the beginning, then a decline," he said.

Radio Advisory episode: Vaccinating the globe, the ultimate systemness challenge

And while antibody levels decrease, they don't completely go away, Bhattacharya said. Some B cells remain in the bone marrow and continue to produce antibodies at lower, but still detectable, levels—and some studies suggest these cells can continue to do so for decades.

Memory B cells also remain in the body over time and are ready to resume antibody production whenever needed. Both types of B cells also continue to develop their immune protections for months after a vaccine or virus leaves the body "in a sped-up form of antibody evolution," Wu writes.

"The quality of antibodies in the body improves over time," Bhattacharya said. "It takes way fewer of them to protect you."

Memory T cells act similarly, hiding out in the body for months as they wait to strike again, Wu writes. T cells are also "more flexible fighters" compared to B cells, which means they can recognize and fight back against viral variants more easily.

Shane Crotty, a virologist at the La Jolla Institute for Immunology in San Diego, said that T-cell responses to Covid-19 vaccines are "exceeding expectations."

However, it can take memory cells a few days to make antibodies after the body encounters a pathogen, Wu writes. This means that if there aren't antibodies already available in or around the airway, the coronavirus may have time to infect cells and even cause some symptoms before a sufficient immune response is available to combat it.

Crotty said that this is "not necessarily a concern," though. According to Crotty, a coronavirus infection unfolds in two phases.

"Initial replication is fast and tough to stop," he said. But more severe symptoms, such as damage to the lungs that require hospitalization, typically take at least a few weeks to occur—which is more than enough time for "even a modest amount of antibodies and T cells" to combat.

Vaccines continue to be effective against disease and death

Over time, even memory B and T cells may no longer be protective against pathogens, though, and this is when protection against disease and death could wane, Wu writes.

Some officials, including CDC Director Rochelle Walensky, have suggested that the current rise in breakthrough infections among the vaccinated indicates that the Covid-19 vaccines may lose their effectiveness against severe disease and death in the future and that additional shots are needed to boost the immune system's memory before it completely fades.

According to CDC data, the Moderna and Pfizer-BioNTech vaccines were around 90% effective and preventing coronavirus infection in the spring, but they are now only around 60% to 70% effective against infection.

However, the Covid-19 vaccines continue to be effective against symptomatic infection, Wu writes. For example, recent studies suggest that the vaccines continue to have effectiveness rates in the 80% range, even against the delta variant.

Furthermore, the vaccines continue to be highly protective against severe disease and death and show little decrease in their effectiveness on these fronts. According to Saad Omer, an epidemiologist at Yale University, there is simply no "evidence of a substantial decline" against the worst outcomes of infection.

And because the vaccines continue to be effective against disease, Jennifer Gommerman, an immunologist at the University of Toronto, said that effectiveness against infection and disease should be considered "absolutely separate."

While a decrease in antibody levels may indicate less protection against infection, it doesn't always mean a loss of protection against serious illness, Wu writes. In fact, even if vaccinated people get infected and get sick, it will happen less often and be less severe, which will make it more difficult for the virus to proliferate and spread.

"The point isn't to protect you from getting even a tiny amount of virus in your body," Müge Çevik, a medical virologist at the University of St. Andrews, said. "That's not what vaccines do."

Instead, Çevik said the goal of vaccination is to "tame the virus" so that it is easier to manage, making infection less of a crisis overall. (Wu, The Atlantic, 9/3; Leonhardt, New York Times, 9/3)