Covid-19: Is it possible to give an indication about herd-immunity?

Part 2

Raymond Meester
4 min readJan 3, 2021

On the 7th of September I wrote on the question: is it possible to give an indication about herd-immunity? To answer this question it’s important to know the real number of infections. In other words we need to know the ratio between confirmed cases and the real cases.

In this previous blog I based the ratio on an American research that said that real infections could be 6 to 24 higher than known cases. Already from the numbers at that time we could conclude that the range from 6 to 24 is far too broad and not very realistic. Based on the current numbers, 24 times would mean that 132% of the US citizens are infected. This is obvious not the case.

We concluded the blog with two conclusions:

  1. More research is needed to find a better ratio between confirmed and real number of cases.
  2. The numbers were very far from 70% threshold and real herd-immunity could only be achieved through a vaccination program.

Meanwhile, we are a few months further. Much research has been carried out and vaccination started in many countries. Let’s focus on one country: The Netherlands.

Total cases in the Netherlands

The Dutch blood bank Sanquin monitors donors with antibodies from March throughout the country. This gives a better indication than the US research. Here are the results of the 26th of November:

The Netherlands has a population of 17,3 million. The average number of people with antibodies in the country was 8,1%. This means around 1,4 million people.

26 November:

Confirmed cases: 503.000

Possible cases: 1.400.000

Ratio: 2.8

Two weeks later, The National Institute for Public Health and the Environment (RIVM) gave the indication (also based on blood tests) that 2 million people had Corona so far.

9 December :

Confirmed cases: 577000

Possible cases: 2.000.000

Ratio: 3.4

These two ratios seem more realistic than they did during the previous American research.

Applying the ratio to other countries

Of course the situation is per country different, but let’s assume this is similar in other countries and apply this ratio on the various countries based on the numbers at the end of 2020 (30–12–2020):

What we can see is that still the numbers are far from the 70% threshold for herd-immunity. At the maximum of possible cases Czech Republic (22,3%) and US (20,5%) have the highest numbers. This is of course still nowhere near the needed 70%. This leaves the second conclusion that a vaccination is the fastest way to herd-immunity.

Herd-Immunity through vaccination

Let’s return to the Netherlands. The 70% threshold for the population of 17,3 million people would mean that 12,1 million people need to be immune. In the last month there were around 10.000 cases per day. Applying our ratio this would mean around 30.000 real cases.

To reach herd-immunity naturally this would mean:

12,1 million citizen = herd-immunity

-2,1 million citizens with a corona infection

— — — — — — — — — — — — — — — — — -

10 million divided by 30.000 = 333 days

At this pace it would take another year, but when daily infections drop to 3000 per day, like it did last summer, it would take another 9 years.

So how fast would the vaccinations be to reach herd-immunity in the Netherlands through that way? We need to vaccinate at least 10 million people (assuming people aren’t contagious after the vaccination, because otherwise herd-immunity would mean that 100% needs to get a vaccination).

The Netherlands starts vaccinating on the 8th of January. Say we want to finish after half a year on the 8th of July. This gives us 182 days. Assuming that there are no vaccination on Sunday, there 156 vaccination days. Because everyone needs two jabs for protection 20 million jabs need to be divided by 156 days. This would mean 128.000 vaccinations per day to reach the herd immunity threshold of 70% (218.000 to reach 100%).

For a year it means 64.000 thousand and 109.000 to reach 100%. This seems to be possible when there are enough available vaccinations: On the following page of the WorldInData we can track all countries on the level of vaccinations: