TThe sheer speed with which Covid-19 spread across the world, along with the novel nature of the virus, has meant that scientists and technicians have been playing a catch-up game. But our knowledge, although incomplete, is now much larger than it was at the beginning of the outbreak, and medical systems are better equipped to respond.
Part of this means that we have improved our disease screening tests, trying to meet the demands of people who want to know if they are or have been infected. Dr. Alex Richter runs the clinical immunology laboratories at the University of Birmingham, where I work, and her team has developed serology tests that detect antibodies against Sars-CoV-2. At the moment, laboratory tests like yours are more sensitive than commercial antibody kits. But an increasing number of the latter are available worldwide, many of them without a prescription.
As we go down this path, it is important for people to understand what antibodies they reveal about our immune response to Covid-19 and how protective immunity works.
Basically, antibodies against Sars-CoV-2 show that you have been infected, although people with other recent coronavirus infections (including some common colds) may have cross-reactive antibodies, giving false positives. Serology tests vary in precision, but are more effective than so-called polymerization chain reaction (PCR) tests, which detect the presence of the virus itself. One reason is that they depend less on the skill of the person doing the sampling: the antibodies in the blood are easier to detect than the virus particles in the nose or throat swabs.
Such tests provide useful information about how an infection has spread through a population. Newly infected people are unlikely to have detectable antibodies for the first few days, but most develop them after one to three weeks. Knowing how many have been infected helps us to estimate the percentage of serious cases and deaths more accurately, and this tells us more about how dangerous the new pathogen could be.
It also helps us identify high-risk groups, which can offer clues to how the virus can be transmitted. Take Richter’s study, which examined 516 healthcare workers for Covid-19 antibodies: 34% of cleaning staff tested positive compared to just 15% of staff working in intensive care. Such ideas can be used to protect vulnerable groups with protection interventions.
Antibodies also offer hope for future protection, but we have yet to fully characterize what Covid-19 immune protection might consist of.
There are two main types of immune “memory” responses: changes in the body that mean you can remember a previous threat to mount a quick protective response on reinfection. The former is powered by B cells, which produce antibodies. Vaccine research aims to generate powerful and long-lasting antibodies that can protect us for life, but this is not always achieved. When the antibodies drop, booster shots can help. When viruses evolve to escape detection, such as the rapidly mutating flu, a newly designed vaccine is needed to stop them in their tracks.
The second type of cell capable of remembering an infection is the T cell. T cells may be sufficient to control infection in the absence of antibodies and to act by organizing immune defenses (so-called “helper” T cells) or directly killing infected cells. to restrict the production of new viruses (cytotoxic T cells). T-cell responses have been detected in most patients with Covid-19, and early human vaccine trials have reported potent T-cell activation. Sars-CoV-T-cell memory may be possible. 2 lasts longer than antibodies, as is the case in other coronaviruses.
Can a T cell response develop without developing antibodies? That seems like a possibility: A small study of patients and their families shows that six of the eight unexpected relatives who contracted the virus at home had T-cell responses but no detectable antibodies.
Currently there is no standard test to measure our T cell responses, and they are not shown in antibody tests. But even a positive antibody test, at this stage, doesn’t tell us much about protection. The duration of any potentially protective immunity remains to be determined. As with so many things related to this pandemic, we will know more as the months go by, and people who have recovered will either stay well or succumb to the virus again.
Perhaps of most practical importance at this time is a study that followed 37 asymptomatic individuals who had positive PCR tests. This showed that they had detectable levels of the virus for longer than those with symptoms. Therefore, asymptomatic people are likely to be more contagious. We also know that pre-symptomatic people, those in the first days after Sars-CoV-2 infection, are also highly contagious. Because we are often unaware that we are infected, measures to ensure social distancing and face coverage are crucial when it comes to protecting others.
However, there is some good news on the horizon. Early data from vaccine studies show that animals can protect themselves from Sars-CoV-2 infection, and people develop distinctive signs of protective immunity. This suggests that immune memory is possible. However, the only way to know for sure is to measure what happens when vaccination volunteers become infected. Ironically, this will take longer in countries that have successfully controlled the virus and have few ongoing infections.
As a viral immunologist, I see immunity in Covid-19 as a puzzle that we have yet to solve. But every week, new posts add new pieces to the puzzle. When vaccines become widely available, protection will be much easier to assess using serological and laboratory tests. At this time, antibody tests do not confirm protection – it is too early to know how much and what would be needed for that. However, they remain important in charting the progress of the virus through our care homes, hospitals, public transportation systems, and workplaces (even if they underestimate those affected). Therefore, it is vital that people who have agreed to be tested and test positive are not disadvantaged in any way compared to those whose status is unknown. Without this type of infection surveillance, much of the difficulties of confinement would have been in vain.
• Dr. Zania Stamataki is a professor and researcher in viral immunology at the University of Birmingham.