Covid-19: Is it possible to give an indication about herd-immunity?
Corona numbers are indicative at best. They give policymakers an indication of the direction in which things are heading. The real numbers depend on many factors like the procedures of registration, the rules of confirmations, the way of testing and many other factors.
A good example is that the confirmed deaths were much lower than the numbers of excess mortality showed. It will be interesting to see this rate compared between countries at the end of the year. Only then we can compare the effects of different policies.
One policy that was quickly abandoned was reaching herd-immunity. Still, some think that we are getting nearer. So what exactly is this kind of immunity:
Herd-Immunity was quickly abandoned as this could take years and may not even work at all. Several studies based on blood samples showed a much lower number of people with antibodies than expected. For example a study conducted in Spain says:
“Just 5% of participants presented with antibodies from point-of-care tests, while antibodies were detected in 4.6% of the blood samples.”
Still, it is believed that there are some pockets in severely affected areas that may have built up some immunity. What counts as herd-immunity has also different outcomes based on models that are being used. For Covid-19 the herd immunity threshold is estimated between 50–83%. In Sweden, they modelled as low as 43%, but based on blood samples in May they were at best at 15%.
Since May however the spread of Covid-19 didn’t stop. Especially in countries like the US, Spain, Peru and Chili there were a lot of confirmed cases. A study in San Francisco based on blood screens estimated the actual cases may lay 6 to 24 times higher. Assuming this is true, how far are we still from herd-immunity based on the current numbers?
The table above let us learn a few things. Even at the highest estimate of actual cases, countries are not at the lowest threshold of herd-immunity. It may be possible that countries like Chile or Panama will achieve some immunity eventually.
Is it possible to give an indication on herd-immunity based on the San Francisco study? No. The range from 6 to 24 times the actual cases is far to broad to give a real indication. Scientists would probably say (like always) that further research is needed. Research that give a smaller range of the actual cases for a specific country. Then based on predictions of new cases we could estimate the time it would still take to reach real herd-immunity.
A new study from Germany gives us not a lot of hope. Current retests of Covid-19 patients from January showed a significant decrease in the number of antibodies. Only those with severe symptoms show enough antibodies to give an immune response. A vaccine shows the only path to real herd-immunity.
Confirmed cases derived on 26–08–2020:
Mortality Analyses - Johns Hopkins Coronavirus Resource Center
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