Perris,
Here are 2 somewhat older studies, that help explain it better, but still line up with what you posted. They pretty much line up with what I was saying, with a distinction in what they are calling immunity, and what I'm calling immunity. What they are calling immunity should be called complete, or total immunity, meaning the recovered person has enough antibodies to completely prevent reinfection. For the most part, total immunity lasts anywhere from 6 weeks to 3 months.
They are not including partial immunity, which they know exists as well. The argument was in regards to how safe it would be for international travel, if a person had a high number of detectable antibodies. This is very misleading, since we know they are using serums with antibodies on people that are at very high risk, and they're working in varying degrees. It's important to include partial immunity as well.
Why is partial immunity important? All of today's modern influenza/flu vaccines do not provide 100% immunity for the viruses they're intended for, on all people. For example, the H1N1 vaccine, which is included in the flu shot, does NOT provide total immunity from Bird Flu for everyone that gets it. About half of the recipients will only get varying degrees of partial immunity, while a lesser number will not get any immunity benefits from it at all. Partial immunity is still a GOOD thing. It means the severity, symptoms, and duration are still reduced, if they catch it. Total lifetime immunity is usually non-existent with most influenza type vaccines. That's why they have to be repeated yearly.
Timing has always been a factor with influenza/flu vaccines too. Immunity is at it's peak shortly after receiving a flu shot, and dissipates over time, until it's time for the next flu shot. This means that being exposed to an influenza covered by the flu shot, shortly after receiving it, will provide a higher level of protection, than being exposed a month before the next flu shot is due. The antibodies being used in the Covid serum, are no different, except they need to be repeated every 2 - 5 months, instead of yearly.
Another thing to consider in these studies, is the inconsistency in the types of antibody tests. This too is mentioned. There is more than one type of antibody test, some being more accurate than others. Without some uniformity in testing, it tends to skew the results. Another thing mentioned, is that the tests have a hard time detecting antibodies below a certain level. This does not mean that they're not there, and offering some degree of partial immunity. Even 40% immunity, meaning the symptoms, and severity will be reduced by 40% can mean the difference between a severe case, and a semi-mild case, and enough to save lives.
As to the information regarding vaccines, their efficacy, yearly deaths in the US from influenza, etc. that information is on the CDC website.
https://abcnews.go.com/Health/covid-19-antibodies-fade-months-study/story?id=71406787
https://www.webmd.com/lung/news/20200622/covid-19-antibodies-can-disappear-after-2-3-months