There are many things that we do not yet understand about the new coronavirus that has driven us all inside and stretched health systems across the globe to breaking point. One of its most fundamental aspects that remains obscure is how many people are actually infected. While official case numbers rise to 2 million, a frequently repeated maxim is that the confirmed cases represent just ‘the tip of the iceberg’. Part of the reason that the number of confirmed cases is so much lower than the actual number of cases is that many people are asymptomatic or present(ed) mild symptoms and have not been tested. Indeed, since testing capacity has been limited across the globe, only people who experience severe illness, or even who end up in hospital, are usually tested for SARS-Cov2 (the virus responsible for COVID-19) with RT-PCR assays. A proposed implication of these asymptomatic or mild-symptom infections is that we could be close to reaching, or even have already reached, a state of ‘herd immunity’. In this state, the immunized population acts as a firewall that protects those individuals that have not been exposed to the virus, disrupting the host-to-host transmission chain.
“Herd immunity is a form of indirect protection from an infectious disease that occurs when a large percentage of a population has become immune to the pathogen. Immunized individuals act as firewalls that disrupt pathogen spreading from host to host. Herd immunity can be reached when the pathogen has infected a large proportion of the population (typically 2/3) or by the generalised use of effective vaccines.”
This is a very enticing prospect, and one that has received a lot of attention in recent weeks. If not backed up by sufficient evidence, however, taking actions based on the principle of herd immunity has the potential to do more harm than good. Slackening of public health measures too early could potentially lead to catastrophic second, and even third, waves of infections, and an overconfidence in herd immunity could detract from ongoing efforts to generate a vaccine or an effective antiviral. Even if herd immunity is achieved through natural infection, we do not yet have convincing evidence that this immunity will keep the virus at bay for long enough and consistently enough to end this pandemic. Before we make any decisions based on this principle, it will, therefore, be important to understand precisely what proportion of the population has been infected, and whether infection confers long-term protection.
“Implementation of systematic serological tests is critical to determine the proportion of the population that has been exposed to the virus.”
The first necessary step will be to develop an accurate serology test to detect antibodies against SARS-CoV2 in our blood. Antibodies are one of our most effective tools in fighting viral infection and are generated as a result of a highly specific adaptive immune response. Hence, the presence of antibodies against SARS-Cov2 is in an unequivocal sign that someone has been infected and recovered from the virus. Once such a test is systematically applied, we will have a better idea on how close we are to threshold for herd immunity to be effective. These analyses will only provide information about the proportion of people exposed to the virus, but will not answer two critical questions: are these antibodies effective at combating the virus, and can they provide long-term protection?
One way in which antibodies can stop being protective is if the virus mutates into a form that is no longer recognised. Unlike other viruses, SARS-CoV2 has a relatively low mutation rate due to a proof-reading enzyme encoded in its genome, which corrects errors introduced by the RNA polymerase during replication. While this does not mean that the virus never mutates, comparisons made between common cold coronavirus strains from 30 years ago and those in circulation today indicate that changes in the viral genome are minimal, and that these do not substantially affect the viral proteins known to be the targets of antibodies. This is important and heartening evidence that a vaccine designed against this virus is likely to confer lasting protection. However, it also poses another question. If the viruses that cause up to a third of common cold cases in the human population are not undergoing significant mutation over time, how is it that people can be re-infected with them every couple of years?
Part of the answer to this likely lies in the quality and longevity of antibody response mounted against these viruses. A study published 30 years ago in which volunteers were infected with common cold coronaviruses found that antibodies declined soon after infection. More worrying is that after a year people could be reinfected when challenged with the same virus. One possibility is that the low pathogenicity of these viruses (most cases of the common cold are relatively mild) results in a half-hearted immune response that is sufficient to clear the virus but fails to generate lasting protection. Evidence that this might be a common problem with coronaviruses comes from a study performed in healthcare workers who recovered from MERS. In this study, authors found that the longevity of the antibody response against MERS was far more variable among individuals who had had mild symptoms than those having severe illness. Since so many cases of SARS-Cov2 infection are thought to be asymptomatic or entailing mild-severity, it is a possibility that a large proportion of infected people lack robust or long-lasting antibody response against the virus.
“Two critical questions remain unanswered: Are antibodies generated against SARS-Cov2 effective at combating the infection? and if so, is the immunization long-lasting?
In a recent pre-print looking at serology of 175 patients who experienced mild symptoms, almost 50% of people had medium or low antibody levels, and 10 patients showed no neutralising antibodies at all. The ability of the antibodies to block infection were assessed through an in vitro neutralisation assay. However, the extent to which this assay correlates with protection in vivo remains unclear. An important aspect that has not been possible to assess yet is how long-lasting this antibody response will be. Hence, whether exposure to SARS-Cov2 will lead to long-term protection is uncertain. Studies with patients infected with SARS-Cov1 (which is closely related to SARS-Cov2) during the outbreak in 2003, indicated that antibody levels declined after 2 years. The risk that natural infection might not provide sufficient long-term protection is an important justification for focusing resources on the development of an effective vaccine and antiviral drugs. By training our immune system with the ‘right’ antigen and following a robust immunization regimen, it might be possible to induce long-term and dependable immunity across the population. Whilst the antibody test will be important in informing policy decisions and epidemiological models in the short term, evidence about the efficacy of natural immunity will critical to identify a long-term solution for this pandemic.