The lethality averages at 0.36% there. That is not too good, but not bad as well.
Still, 0.36% would mean 250,000 deaths in Germany this year until we reach a possible herd immunity, ten times more than a bad influenza season. We should better hope for a vaccine.
The 0.36% are actually the same level as a moderately severe influenza strain, falling below e.g. the Hongkong Flu (which has a mortality rate of 0.5%).
In order to get 10 times the number of dead than for influenza with the same mortality rate as influenza, you need to assume that 10 times the number of people will be infected. Indeed, doing your numbers with a mortality of 0.36%, to get 250.000 dead you have to assume that about 70 million Germans (or 83% of the population) will be infected.
Now, even the naive herd immunity criterion using R0 doesn't end you that high - that gives you about 67% of the population.
But perhaps more telling, the same Heinsberg study also found that the risk of someone living in the same household than an infected person to get the virus himself never goes above 40% (the study claims a dependence on the number of household members with it going down when more people share the same household), in other studies that number came out more around 20%. All of that is very much in line what has been seen on the USS Theodore Roosevelt (with 4500 people living together in closed quarters).
So in order to get to the estimate of 83% of the population becoming infected, you need to assume that chance encounters and conversations on the street are
vastly more contagious than actually living together with an infected person (i.e. basically being exposed to the virus all the time).
Once you assume that the total percentage of infections cannot exceed what you see as spread inside a household, you're down to an estimate that no more than ~30% of the population will be infected in the end. If you assume that chance encounters are actually less dangerous than sharing a household, you're getting the feeling that the 15% which have happened in Heinsberg is probably more or less all that will happen there.
You have to introduce quite some heavy assumptions to get to a number of 250.000 dead in Germany by now...
Maybe also interesting: Only one fifth of all infected people had no symptoms at all (including loss of smell and taste as symptom there) - the older number, that about 80% of the cases might be asymptomatic, does not hold water there.
At this time of the year, perhaps a good question is how many of the 85% who did not contract the virus did still have symptoms like cough, sore throat or even a little fever. Heinsberg happened during the Influenza season, and even the common cold causes things like cough.
