Covid-19 Immunity

The World Health Organisation has said that there is no evidence that people who have recovered from a coronavirus have immunity to the disease. 

They have, however, said that the antibody tests will be able to measure the level of seroprevalence, i.e. the level of antibodies, but this does not mean that someone with antibodies is immune.

It is unclear where COVID-19 falls within the immunity spectrum.

Most people with COVID-19 seem to produce antibodies says Dawn Bowdish Professor of Pathology and Molecular Medicine at McMaster University in Ontario.  However, we simply do not yet know what it takes to be effectively protected from this infection.

It appears that recovered COVID-19 patients have antibodies for at least two weeks but long term data is lacking. 

If we look at other coronaviruses for answers, immunity to the common cold (seasonal coronaviruses) declines a couple of weeks after infection and within a year some people are vulnerable to re-infection (this is of course concerning as it is unlikely that we will have a vaccine for COVID-19 within the next year). 

Studies of SARS-CoV, the virus that caused Severe Acute Respiratory Syndrome (SARS) show that the antibody peaks at around four months and offers protection for about two to three years as researched by Preeti Malani, Chief Health Officer and Professor of Medicine at the University of Michigan.

However, even if antibodies are in the body, it is not yet certain whether they will prevent future infection.  What is required are neutralising antibodies.  These are proteins that reduce and prevent infection by binding to the part of a virus that connects to unlocked host cells.  They are easy to detect and are far easier for vaccine developers to generate.  Non-neutralising antibodies still recognise parts of the virus antigen but they do not bind effectively and do not prevent the virus antigen from invading cells.

Bowdish continued that if humans make neutralising antibodies against the
COVID-19 virus then researchers need to discover at which sites they are binding onto the virus and then target that part of the protein.  It is most likely the target is the so called receptor binding domain of the spike glycoprotein.  This protein is attached to a sugar and this has become a challenge as the human immune system is not good at making antibodies against sugar quoted substances.

Encouragingly, a few small studies of cells in the laboratory suggest that the
COVID-19 infection triggers the production of neutralising antibodies.  Animal studies indicate that these antibodies do prevent re-infection at least for a couple of weeks.  Researchers can build on knowledge from previous attacks with MERS and SARS.

Research is in its preliminary stage and remains uncertain.  However, worryingly one study found no correlation between viral load and antibody presence which led the researchers to question whether the antibodies play a role in clearing the virus in humans.  Peer reviewed research on COVID-19 reported that some non-neutralising coronavirus antibodies might actually trigger a harmful immune response upon re-infection.

Therefore, currently antibody testing of populations cannot drive policy until researchers know the proportion of COVID-19 survivors who are producing neutralising antibodies. 

Eventually, antibody tests will be the key to getting our lives and economies back on track. 

The London General Practice

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