We’ve been hearing talk about herd immunity quite a lot recently. Is this our light at the end of the COVID-19 tunnel or just more media hype?
In this post, I attempt to get a more realistic picture of what’s it actually means to achieve herd immunity, and what it takes to achieve it.
What is Herd Immunity?
Herd immunity occurs when a certain percentage of a population has become immune to infection by a given pathogen, in this case SARS-CoV-2, the virus that causes COVID-19. Once the threshold has been reached, the chain of infection is disrupted enough that even individuals who are not immune to infection enjoy a reduced risk of catching the disease. In practical terms, it is the point at which community spread has dropping enough that we can stop wearing masks in public.
Immunity comes from the presence of antibodies which are acquired either by vaccination (which currently doesn’t exist) or by prior infection.
How do we achieve Herd Immunity?
For your garden varieties of influenza, it takes between 33 and 44 percent of the population to be immune before herd-immunity status is achieved. That most certainly does not apply to SARS-CoV-2 because it is so highly contagious.
The Johns Hopkins article, Early Herd Immunity against COVID-19: A Dangerous Misconception, suggests that we need 70% immunity to achieve herd status. I personally feel that is a little too optimistic.
To find out how they come up with this percentage, we need to look at how the spread of infectious diseases is modelled. Using this formula we find that higher the value of the “basic reproductive number” ( R0 ) of the disease, the higher the threshold for achieving herd immunity. If R0 =1, then one infected person will, on average, infect only one other person. Of course, the more contagious the disease, the higher the value of R0 .
The Immunity Journal article, Herd Immunity: Understanding COVID-19, suggests that the value of R0 for SARS-CoV-2 is between 2 and 6. If we assume an R0 value of 3, the herd immunity threshold would be about 67%, slightly more optimistic than the Johns Hopkins article suggests.
The article warns, however, that these values assume there is a homogeneous mixing of individuals within a population and that all individuals with the antibodies enjoy a lifelong protection against reinfection.
Why we should not be too optimistic
New York City (population 8.3 million) has been hit pretty hard, with almost 223 thousand confirmed COVID-19 cases (2.7% of the population) and nearly 23 thousand deaths (0.2% of the population), so far.
… if we (optimistically) assume the COVID-19 death rate is only around 1.3% … the total cases (and the total deaths) will triple before herd immunity kicks in.
As hard as NYC was hit, 2.7 percent is a long way off from even the most optimistic notion of herd immunity. It would take 5.56 million New Yorkers to be immune to the virus, before reaching the 67% threshold. That would be a 25-fold increase, if you base it on confirmed cases of COVID-19!
To be fair, the confirmed cases are always much lower than actual cases. As such, the 10.3% death rate is exaggerated by the discrepancy between confirmed and actual infection rate.
As time goes on, the confirmed COVID-19 cases should continue to rise dramatically as more people get tested, but the death rate as a percentage should significantly drop, as we increase our SARS-CoV-2 testing and refine our treatment protocols for severe cases.
So, if we (optimistically) assume the COVID-19 death rate is only around 1.3% of the actual infections, then the total NYC infections would be about 1.7 million, or 21.3% of the population. So, the total cases (and the total deaths) will triple before herd immunity kicks in.
Is immunity even possible??
While widespread antibody testings shows promise for giving us a more accurate picture of the infection rate, there’s still a lot we don’t know about SARS-CoV-2 antibodies, or their ability to impart immunity at all. If we model using data from other forms of coronavirus, we are still looking at only one and two years of immunity before it declines and people start getting reinfected.
We are also seeing cases were recovered COVID-19 patients are testing negative for any coronavirus antibodies at all. This suggests that the innate immune system is kicking ass before the adaptive immune system can respond, or that the adaptive immune system is somehow compromised. Either way, it doesn’t bode well for immunity from reinfection.
Assuming antibodies impart any kind of immunity to infection or reinfection, the only way to achieve herd immunity (while keeping the mortality low) is to develop an effective vaccine. And with the expected decline of immunity over time, annual SARS-CoV-2 shots may well become a routine part of our annual flu-shot regimen, making the whole herd immunity discussion moot.
On the bright side, remember that 80% of the COVID-19 cases are non-critical which means no hospitalisation at all. The best way to stay out of the 20% high-risk group is make the lifestyle and dietary changes I have been tooting my horn about for 20 years!
- keep your hands clean/sanitised
- don't touch your face without first cleaning/sanitising
- practise physical distancing
- completely cover your coughs and sneezes
- and finally, wear mask!!!