Antibody levels will taper in the months following vaccination or infection, but that doesn’t mean they plummet to zero, Bhattacharya told me. Although most of the B cells die off, some stick around in the bone marrow and keep churning out the virus-fighting molecules at more modest, but still detectable, levels. Though the life span of these long-lived B cells can vary, some studies have hinted that they’re capable of persisting as antibody factories for decades. Another population of immune cells, memory B cells, meanders around the body like sleeper agents, ready to resume making its antibodies whenever necessary. All of these B cells can continue to broaden and intensify their virus-vanquishing powers for months after a vaccine or pathogen leaves the body, in a sped-up form of antibody evolution. “The quality of antibodies in the body improves over time,” Bhattacharya said. “It takes way fewer of them to protect you.”

Populations of memory T cells, too, can hide out for many months or years in tissues, waiting to strike again. Although antibodies are very picky about what they attack, making them easy to stump with viral mutations, T cells are more flexible fighters that are great at recognizing variants. Shane Crotty, a virologist at the La Jolla Institute for Immunology, in San Diego, and his colleagues have documented durable T-cell responses to COVID-19 vaccines. They’re “exceeding expectations,” he told me. “Overall, it looks like there’s high-quality memory at six months.”

Memory responses take a few days to get going. That’s far faster than the response to a first inoculation, when B and T cells are naive to the threat. But if antibodies aren’t already lurking in and around the airway, the virus might get a chance to invade a few cells, maybe even cause some symptoms, before sufficient reinforcements arrive. That’s not necessarily a concern, said Crotty, who described SARS-CoV-2 infection as unfolding in two phases. “Initial replication is fast and tough to stop,” he said. Severe, hospitalization-worthy damage in the lung, however, tends to take at least a couple of weeks to manifest—plenty of time for “even a modest amount of antibodies and T cells” to interfere.