David G. Bostwick MD, Rebecca Caffrey PhD, and Nicole Waltrip MS
Why is antibody testing considered necessary by public health officials? Such testing can’t even tell you if you are currently infected and shedding virus; however, it can determine if you were infected. Concerns have been raised about inaccurate testing and false results. On May 4, the FDA weighed in, stating, “…we unfortunately see unscrupulous actors marketing fraudulent test kits and using the pandemic as an opportunity to take advantage of Americans’ anxiety.” In response, the agency strengthened testing to require overall sensitivity of 90% and specificity of 95%. This action will likely improve testing accuracy quickly.
There is good news for those who want to know if they were infected, including sick patients who were not able to get tested, those who suspect their swab test was a false negative, and those who were exposed to COVID-19. A recent study from China reported that antibody testing provided definitive evidence of prior infection. Investigators found that all 285 infected patients under study developed antibodies within three weeks of onset of symptoms, suggesting that the immune system of survivors invariably develops a response specific for COVID-19.
Communities benefit from antibody testing by determining the extent of asymptomatic and symptomatic infection, tracking of infections geographically and over time, estimating the prevalence of infection in the population, and identification of recovered patients with antibody titers high enough to donate convalescent plasma to treat the sickest patients currently suffering from infection.
Those with antibodies can serve as donors to help save lives of infected critically-ill patients. Infusion of antibody-rich convalescent blood serum into sick patients has been successfully used for more than 130 years in numerous outbreaks, including diphtheria, 1918 Spanish flu, measles, chickenpox, SARS, and even Ebola. Such treatment exploits pathogen-specific protective immunity conferred by antibodies, and multiple recent studies have demonstrated significant clinical improvement and decreased mortality in seriously ill patients within a few days.
Yet, we still need confirmation that antibody-mediated virus-specific immunity can protect against re-infection, and we need to know how long it will last. The duration of immune protection is known for most other viruses. For example, antibodies to measles last a lifetime, whereas patients infected with another coronavirus—SARS—lose all antibodies by six years. At present, we can only surmise that the development of antibodies in COVID-19 infection confers some level of immunity.
The only real solution for reopening our world and returning to normalcy is to develop herd (community)immunity, and this can only be measured by antibody testing. The concept of herd immunity is grounded in scientific studies that the greater the number of people who are immune to a given pathogen, the fewer that can be infected. Recent surveys show wide variability in antibody prevalence, ranging from about 4 percent in Los Angeles, 25 percent in New York City, and 31.5 percent outside Boston. All are far short of the estimated 60-85 percent needed for COVID-19 infection herd immunity.
As we strive to reach herd immunity, whether though widespread recovery from infection or future vaccination, antibody testing will play a vital role. All agree that we will recover from a pandemic that spawned the largest and only healthy-person quarantine in history by taking the only medicine that is currently available to all of us: “a tincture of time.”