But these figures are likely an underestimate, the findings of a new Centers for Disease Control and Prevention (CDC) report suggests.
Among more than 150 health care workers who had tested positive for coronavirus, more than a quarter had no detectable antibodies 60 days after diagnosis, according to the study, published Tuesday.
We still don’t know whether detectable antibodies are necessary for protection from reinfection, however. In fact the latest research suggests that other parts of the immune system do ‘remember’ coronavirus, months after recovery.
However, it’s still worrisome because it could mean that cities and states are not getting an accurate picture of how widespread coronavirus is or has been in their communities, which could skew policy decisions like whether to shut down non-essential businesses.
In total, 28% of the health care workers who initially tested positive for coronavirus no longe had them at detectable levels 60 days later. Nearly 70 percent of those who had low levels to begin with (top) had effectively none two months later, the CDC found
During that time, 2,425,932 coronavirus cases were recorded in the U.S., according to data from Johns Hopkins University.
And by the end of June, at least 2.6 million Americans had had coronavirus.
It’s assumed that daily and total infection and death tallies are undercounts, but data from antibody testing – called seroprevalence surveys – are meant to help fill in the blanks of how many infections might be missed.
An active infection is only detectable for a brief period of time in most people, with symptoms and viral loads dissipating to undetectable levels in about half of patients within 14 days.
Diagnostic tests only pick up the virus when the infection is active and there is enough viral genetic material in a person’s mucus to be detected.
But antibodies form over the course of infection and stick around longer – although for how long is still clearly the subject of much debate and research.
So antibody testing was looked to as a way to quantify not just how many people currently have active coronavirus infections, but how many might have had the virus without knowing it or be counted among the positives.
The CDC currently says that antibody testing is probably not useful 90 days after someone thinks they may have been infected.
However, the new study published by the agency suggests that antibody testing may serve little purpose even earlier.
The researchers screened 3,248 health care workers for a history of COVID-19, its symptoms and contact with covid-positive patients.
Of the whole group, six percent had antibodies to the virus upon their first visit.
Only 156 participants returned for a follow up visit 60 days later.
Seventy percent of that group said they’d had at least one symptom of COVID-19 since February. Nearly a third had tested positive for an active infection in the past.
By the time of their 60-day follow appointments, antibody levels had declined in almost everyone who had developed them (93 percent).
More than 28 percent no longer had detectable levels of the immune proteins.
Antibodies were far more likely to vanish in people who had never developed symptoms, of whom nearly half lost detectable immune cells.
Levels also fell too low for the highly sensitive test to flag in more than 19 percent of those who did develop symptoms.
‘Seroprevalence studies that estimate the number of persons who have been infected with SARS-CoV’ – the virus that causes COVID-19 – ‘ will likely underestimate incidence because a proportion of previously infected persons will likely serorevert and thus not be counted as having been previously infected,’ the study authors wrote.
They added that the findings call into question whether antibody testing is even a useful way to tell individuals whether they’ve had coronavirus, much less whether they can get reinfected.
And it also means that supposedly antibody-rich plasma, may not be so rich in those immune cells for long.
‘These results demonstrate that the optimal window for collecting convalescent plasma with high levels of SARS-CoV-2 antibodies from donors who have recovered from COVID-19 might be short because of substantial decline in antibody levels within 60 days.
‘Whether decline in SARS-CoV-2 antibodies increases risk for reinfection and disease in humans remains unknown. Humoral immunity to primary infections from a novel virus might not be as durable strong as that to secondary infections, but memory B-cell and T-cell responses might reduce the severity of illness with repeat exposure or infection.’