Tests for Covid-19 antibodies are now available, but news reports suggest—and some physicians are advising their patients—that antibodies may not provide immunity. This assertion defies generations of immunology research and is a political attack on reliable tests.
Here’s how the immune system works: On viral infection, it reacts with two surges, “innate” and “adaptive” immunity. The innate response comes within minutes to hours and triggers alarms that result in effects across the body such as fever. Tissues and cells produce “interferons,” molecules that incapacitate many viruses and recruit white blood cells.
For mild infections, innate responses are sufficient to defeat the foe. But some viral infections require a second wave of response, and adaptive immune responses arrive four or five days after infection. Molecular bits of the offending pathogen, known as antigens, are brought to the lymph nodes, where white blood cells called T and B lymphocytes respond. These lymphocytes head out to the front lines—the infected tissues, such as the lung for Covid-19. The wave of T cells that arrive at the battlefront deploy the principal weapon in their arsenal, the release of cytotoxins, to kill virally infected cells. The over-aggressive immune response causes much of the devastation in severe cases of the disease.
Meanwhile, B cells pump out antibodies that over several weeks adapt to the pathogen. After the war is over, a few T and B cells linger in the lymph nodes and in the mucosa of the airways, forming an “immunological memory” that is programmed to fight faster and stronger the next time that pathogen shows up. Such cell memory provides “protective immunity,” which Thucydides first hypothesized in 430 B.C.
Antibody tests come in two broad forms. One is the lateral flow kit, which works like home pregnancy tests and can be used at a doctor’s office for rapid results. The other is the enzyme-linked immunosorbent assay, or Elisa, which are typically run in a lab. Used appropriately, both of these antibody tests can provide valuable information. A lateral flow kit is rarely as sensitive as the lab test. But in a survey looking for antibodies as evidence of past infection in hundreds or thousands of people, the test is reliable enough to yield information about the progression of the pandemic, the infection rate in a population, and the fatality rate.
The laboratory-based tests authorized by the Food and Drug Administration for emergency use—those from Abbott, Ortho Clinical Diagnostics, Roche and others—are excellent. Several tests have published sensitivity and specificity values as high as 99.6% to 100%. These tests are consistent with other antibody tests, such as those for mononucleosis and hepatitis infections, that are in routine use without much concern about their accuracy.
If these antibodies aren’t protective, then global efforts to develop a vaccine are pointless. Vaccines try to arm T and B cells so they fight quickly when exposed to the virus. If antibodies detected in a person who has recovered don’t confer immunity, then neither would antibodies developed in response to a vaccine. The far more likely scenario, which is true of other coronaviruses, is that antibodies do offer protection for a significant duration, so that a successful vaccine could be developed.
Fear has arisen from the finding that antibody levels fall over time. But that is observed in every response to infection or vaccination. The memory of B cells means the body can produce antibodies when needed in the future. If levels of protective antibodies didn’t wane rapidly after infections, the blood would become thick with antibodies over a typical winter season. What isn’t known: Will immunity last a lifetime? Or, like tetanus, will it require a booster a decade later? Or will it require a seasonal shot as new strains emerge, as with influenza?
Still, you should consider talking to your physician and getting tested for antibodies if you suspect a prior infection, or if you wish to participate in a seroprevalence survey. Our understanding of the disease has been improved by seroprevalence studies in cities like Robbio, Italy, and Gangelt, Germany, and in states such as California and New York. Many other such antibody studies are under way. Furthermore, antibody testing is the only way to identify convalescent serum donors for treatment of severely ill patients, and it is also required to qualify for some vaccine trials.
So far the data have shown Covid-19 is more widespread and thus less lethal than previously thought. More antibody data will enable better policy. These accurate and sensitive tests should be made widely available, not disparaged.