One of the main challenges in response to the COVID-19 pandemic has been testing. Diagnostic tests can confirm if someone has an active COVID-19 disease, whereby antibody tests confirm if someone previously had the disease and has developed immunity. Numerous diagnostic tests have been developed to improve the accuracy, shorten the time required to produce results, and make the process to obtain a sample less invasive. Abbott has produced a diagnostic test that requires only 5 minutes and a sample from the nostril, which is faster and less obtrusive than previous tests obtained from nasopharyngeal samples. Diagnostic tests allow medical providers to make a differential diagnosis, recommend self-quarantine and pursue supportive care measures. Recent CDC statistics estimate nearly 5 million people have been tested for COVID-19 in the United States.
Another significant type of testing is serology testing. Dr. Redfield, the CDC Director, has said that these tests will be an essential part of the United States plan to move forward. The purpose of serological tests is to determine the presence of antibodies in the blood. Antibodies develop as part of an immune response to infection, as opposed to diagnostic testing which aims to detect the presence of a disease-causing agent, known as an antigen. Serological tests can provide insight into how many asymptomatic people have been previously exposed to the virus and have developed antibodies to fend off infection, and possibly prevent new infections. The presence of these antibodies may suggest people could return to work in a safe manner.
The National Institutes of Health (NIH) has launched a study utilizing antibody testing in over 10,000 people to quantify undetected cases of COVID-19. Quantifying asymptomatic cases and those that have immunity may shed light into the degree of virility and transmissibility of the virus. Therapeutically, serological tests may be utilized to determine who can donate blood that can be used to manufacture convalescent plasma as a possible treatment for critically ill patients with COVID-19.
Epidemiologists often reference “herd immunity” when discussing communicable diseases. Herd immunity is an indirect protection from infectious disease that occurs when a large percentage of a population has become immune to an infection. Thus, providing a measure of protection for individuals who are not immune. How contagious a virus is determines what percentage of people need to be immune to achieve herd immunity. To put it into context, measles requires a minimum of 92% of a population to be immune to prevent its spread. Laboratory data has indicated SARS-CoV-2 is not as contagious as measles. Thus, according to researchers at Johns Hopkins University, if 70% of a population is immune to COVID-19, herd immunity may be achieved.
There are still several questions that remain regarding antibodies and immunity against SARS-CoV-2. The length of immunity is unknown and may play a crucial role in establishing guidelines for reopening the economy, relaxing social distancing requirements, better understanding the immune response to the virus, and vaccine development. Length of immunity can vary widely as observed previously with other viral infections. For example, immunity to the common cold can be as short as a few months, while protective antibodies against the virus that caused the SARS outbreak in 2003 are still present in people almost two decades later. The length of immunity will help to model a potential vaccine schedule like an annual flu shot or a tetanus vaccine administered every ten years.
The immune system develops several different types of antibodies in response to an infection; however, it remains to be determined which antibodies should be the primary focus. IgM is typically the first antibody produced by the immune system, followed by IgG. Testing for IgM can indicate a more recent infection, while the presence of IgG may be a better indicator of sustained immunity. A potential area of concern is that if a person is tested shortly after coming in contact with the virus and has not yet had time to develop antibodies, test results may indicate a false negative. It is not currently known how long antibodies will remain present in the body after the infection has been resolved.
The FDA has authorized an expedited process for the many antibody tests in an effort to increase accessibility. Only four of the -more than 90 antibody tests now on the market in the U.S. have gone through vetting through the Emergency Use Authorization (EUA) process. The biggest concern is that tests may not have a high level of specificity; they may return false positives when someone has developed antibodies for other viruses. Due to the variables and current limitations of antibody tests, the FDA has issued recommendations to health care providers -not to use these antibody tests as the sole basis to diagnose COVID-19, but rather as information about possible prior exposure.
In the constantly evolving battle against COVID-19, serological tests will provide an essential tool for scientists and medical professionals as more data is compiled and tests become more refined.