[EvaH (OP)]

Paul wants to know:

If you catch Covid-19 from someone who is on, for example, day 8 of being infected (and has fairly mild symptoms), are you then catching a weaker version of the virus because the person you caught it from has started to defeat it? Are you catching a less threatening version?

Can you help?

[alancalverd]

Not necessarily. If I can pick up 100 kg with one hand, are they lighter kilograms than average?

[Bored chemist]

Not necessarily. If I can pick up 100 kg with one hand, are they lighter kilograms than average?

I think this is a better analogy.

If a strong person gives you a heavy package, does that make it easier for you to carry?

The effect of  the virus on you depends on you (and the virus), not the person you get it from.
It is possible that they had (and you will get) a weak variant of the virus but even then "weak" isn't well defined, the same bug affects different people in different ways.

[evan_au]

are you then catching a weaker version of the virus because the person you caught it from has started to defeat it?

The reason the other person has started to defeat it is because their immune system is now producing large quantities of antibodies which disable the virus from infecting more of their cells (and many other parts of their immune system are also in high gear).
- This is something the other person is doing
- It does not imply that the virus has changed in any way - they may still have the same strain of the virus with which they were originally infected.

However, your immune system is not familiar with this virus, and is totally unprepared for it (the term is "naïve")
- That means that if you are infected by this virus on Day 4 or Day 8 or Day 12 of the other person's infection, the virus will attack you in the same way
- One difference may be that on Day 4, the other person is shedding lots of virus, but by 12 they will probably be shedding much less
-Getting multiple simultaneous sites of infection is likely to lead to worse outcomes for you, since the virus has more opportunity to "take over" before your immune system reacts

Since you never know whether other people are infected or not, wear a mask, and stay socially distanced (until a month after your second vaccination)
- And for the peace of mind of others (who don't know that you have been vaccinated), continue taking those precautions until most people have received their second vaccination, and infection rates are very low (achieving herd immunity).
- After all, the best vaccines are only 90% effective at preventing infection, and you don't know if you are in the 10% who are still susceptible
- With some of these new variants, the effectiveness will be even lower.