I know a person who had mild symptoms, was tested positive in the PCR test and did not have any antibodies 6-8 weeks later. He then looked into the subject deeply. His conclusion was: You can develop antibodies after infections, but not everybody does. Also: a lack of ABs does not mean that you aren't immune. After all, many people were asymptomatic or showed only mild symptoms without any ABs... There is the second, much less known immune system called cellular response, which seems to play a strong role in this disease.

Isn't it also possible that the PCR test was wrong and it was something else, like a cold?

The PCR test is a very specific test. A positive test means the RNA sequence of SARS-CoV-2 was detected. A negative test, however, would not be as strong of an indicator as false positive rates are quite high early or late in the infection (https://www.biotechniques.com/coronavirus-news/news_false-ne...).

As far as I know a false positive in PCR is not very likely, but of course it is possible.

PCR is very unlikely to give a false positive if you never had the virus. The viral RNA simply isn't present to be detected. A false positive from PCR most likely came from contamination of the test materials.

PCR is quite likely to give a false positive if you previously had the virus but have recovered, including asymptomatic cases. There are still pieces of viral RNA that can be detected by PCR, even if they're inactivated by having destroyed the viral coating and you're no longer contagious.

So PCR is pretty accurate at saying if you ever had the virus, but not so accurate at saying if you currently have it at contagious levels.

Or the antibody test was wrong. Seems most of the tests look super noisy.

I'm under the impression that not much confidence can be placed in the results of a single AB test.

I'm not sure if it's the case but maybe 60% is the possible maximum?

I hope I understand it correctly how germs spread. Is it like this?

Imagine an infected person infects 4 other persons, and the incubation period was 1 year. Probably everyone would get the germ.

But if the incubation period was 1 second, at a certain point 1 infected person would find less than 1 person who is infectable (not already infected / had an infection that is gone now / has had an immunity? / etc.).

I posted elsewhere, but yes, there are thresholds for transmission when enough of population has developed immunity: https://en.wikipedia.org/wiki/Herd_immunity#Mechanics

Herd immunity halts transmission after a point. The flu is between 33% and 44%. Covid-19 has some wide estimates for the threshold, between 29% and 74% according to the two studies referenced here: https://en.wikipedia.org/wiki/Herd_immunity#Mechanics

Herd immunity and r0 would be significantly different on a ship

In the french air carrier CDG , 1,081 out of 2,300 were found infected

https://en.wikipedia.org/wiki/COVID-19_pandemic_on_Charles_d...

Not everyone showed up for testing. Let's also not forget that the antibody tests are proving to be less than accurate. A high number of false negatives.

As far as I remember that's the case with PCR, but not with the AB tests. They rather show a high number of false positives...

I am fully aware that this is non-scientific, but I'm in a covid 'survivors' group of ~52k people on FB.

What I'm seeing there is very few people saying they tested positive for AB and a whole lot more who say they know they had it and tested negative.

Truth is, we just don't know either way. =(

Update: This was a good post from the covid group, sounds like people also just don't develop enough antibodies to show up on tests... https://medium.com/@mrina.nair/my-familys-covid-experience-2...

That’s really weird. But it fits the pattern my friend observed. Tested PCR positive with mild symptoms but negative for ABs...

Wouldn't that imply an R0 of 10 which is much higher than generally estimated? It should stop once it reaches her immunity and possible sooner if quarantines are successfully placed and the infection dies off. 60% sounds like about what you'd expect, or even a bit high.

Couldn't it reach such a percentage with a R0 being lower than 10?

If your R0 is 2.5, herd immunity will make the current R value go under 1 when 1 - (1 / 2.5) = 60% of the population got infected.

While that means that the epidemic will stop growing exponentially, the virus will not stop propagating, as there will still be a non-exponential propagation of the virus (e.g. if 10% of the population is still infectious at the time herd immunity is reached -- when R becomes 1 --, we should expect at least 10% more infections). Or am I missing something?

R0 on a ship will always be higher than in pretty much any other situation. Most ships are packed to the gills, and in a gigantic ship like a carrier it will be even worse, since most of the crew will be in poorly ventilated areas rather than on deck.

I believe this was the ship where the captain tried hard to intervene in the spread and got lots of the sailors off the ship to avert the spread.

Also worth noting is that this is the CDC performing serology tests in April, when the antibody tests were not performing terribly well.