This is the first article in a four-part series on fading immunity to Covid-19 and the flu—the science behind it, and the implications it has for vaccine development and the pandemic at large.

More evidence has emerged that immunity to Covid-19 is quick to fade—in people of all ages, but more so for the old than the young.

A recently published British study, currently undergoing peer review, found that the prevalence of Covid-19 antibodies across England has dropped more than 26 percent in three months. The results come from 365,000 home tests that were sent to participants in three rounds, distributed randomly each time.

Antibody testing, which measures the proteins your body produces to ward off a specific pathogen rather than live virus, has been used throughout the pandemic to pick up on infections that standard PCR tests miss. This is especially useful in detecting so-called silent spreaders—the asymptomatic carriers who don’t get tested because they don’t get sick, but may cause as many as 80 percent of infections.

The most precipitous decline in antibodies observed in the British study, in fact, was in people who didn’t report a history of Covid-19 symptoms—in other words, asymptomatic participants. Antibody levels in this group dropped nearly two thirds between the first round of testing and the last, while those in the people who tested positive for the disease decreased by a bit more than 20 percent.

Another notable drop was seen in a population that, if these antibodies are protective, need them most: participants 75 years of age and older. Their antibodies decreased by nearly 40 percent. In participants belonging to the youngest age group (18-24), the decline was much smaller at about 15 percent. Only in one group—health workers—did rates hold steady; this may be due to their increased exposure to the virus.

The British study is the latest to join a growing body of research that arrives at the same conclusion—that the antibodies we produce against SARS-CoV-2, the virus that causes Covid-19, are short-lived. One study, published in Nature Medicine in June, measured antibody levels in a group of 37 Covid-19 patients three to four weeks after initial infection, then two months thereafter. Surprisingly, about 20 percent tested negative for antibodies altogether—indicating either total disappearance or levels so low as to be undetectable.

These findings were echoed in another study published in JAMA last month on healthcare personnel who work directly with Covid-19 patients at Vanderbilt University Medical Center in Nashville, Tennessee. Nearly 60 percent of those who tested positive for anti-SARS-CoV-2 antibodies in April tested negative only two months later. Again, their antibody had levels dropped so sharply, they were no longer detectable.

Since it is unknown whether these antibodies are even protective against the virus in the first place, it is difficult to say with certainty whether the swift dissipation of the antibody response leaves the body vulnerable to reinfection. But it would hardly be surprising if it did. Besides occasionally morphing to become highly transmissible and lethal threats to humankind, seasonal coronaviruses have another signature move—their ability to reinfect the same person not just once, but two or three times, as documented in research from the 1970s and 1980s and a much more recent 2018 study conducted in Kenya.

There have been reports that the T cell response against coronavirus infections, including those caused by SARS-CoV-2, is longer lasting than the antibody response. T cells are a type of white blood cell that, like antibodies, are critical to our ability to prevent subsequent encounters with a harmful virus. In the case of seasonal cold-causing coronaviruses, however, T cell memory has little to no effect in preventing reinfection, which can occur yearly like clockwork. An absence of protective antibodies doesn’t augur well for T cell immunity, no matter how you spin it.

A handful of Covid-19 reinfections have already been confirmed via genome sequencing in Hong Kong and the United States and reported anecdotally in Europe, India, and South Korea. Now that a second wave of infections has overwhelmed Europe and a third is crashing over the United States, we’d be remiss to dismiss the possibility that subsequent waves of reinfection might exacerbate present and future outbreaks. Not only does this all but dispel the dangerous fantasy of herd immunity, it also complicates a near inevitability that actually looms large on the horizon—the coincidence of the ongoing Covid-19 pandemic with the arrival of seasonal flu.

Antibodies against Covid-19, it turns out, aren’t the only ones to dwindle rapidly. Those produced by standard influenza vaccines, new studies show, disappear at a similar clip. I will explore the implications of this finding—both in the context of influenza research and the current health crisis—in the next part of this series.

This article originally appeared in Forbes and is available online here: New Study Offers More Evidence That Immunity To Covid-19 Fades Quickly