[crystal_magic (OP)]

I am looking to get my Covid booster jab, but I am currently sharing a house with someone who is isolating after testing positive 3 days ago. If I am showing symptoms I wont get one but does receiving a booster jab whilst being asymptomatic reduce its efficacy or possibly increase the risk severe illness?

[evan_au]

If your flat mate tested positive 3 days ago, you have been exposed for around 6 days. You have either fought it off (with help from the vaccination) or are fighting it off (with help from the vaccine). Delta is very infectious (the jury is still out on Omicron).

I suggest that you first get a PCR test to see if you are infected, and if not, get the booster.
- The people taking COVID swabs are acting like they will be exposed to COVID-19
- The vaccination clinic is not so prepared, and will say "Don't come if you are infected, have been exposed, or are feeling any symptoms".

I heard some preprint research that suggests that vaccination within 2 weeks of infection can reduce the chances of Long COVID significantly, so don't wait too long.

Of course, if your flatmate has Omicron, preprint in-vitro results suggest that the vaccine won't be nearly as effective as it is against Delta.

[set fair]

but does receiving a booster jab whilst being asymptomatic reduce its efficacy or possibly increase the risk severe illness?

I don't think there's any evidence of that. Do get tested, if you're positive, waiting 2 or 3 months for a vaccine does give very good immunity.

[CliffordK]

A little late now, but testing isn't bad.  Then I'd go ahead and get the booster.  Hopefully at a drive-through clinic or similar that would have minimal exposure for others who are more vulnerable.

There are some diseases like Rabies that can be effectively vaccinated during the latency period.

COVID isn't quite the same, but it wouldn't hurt to wake up the immune system shortly before the disease hit.