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Comment by curiousgeorgio | Hacker News Reader

Question: Could a COVID-19 vaccine (or any vaccine for that matter) have contagious immunity? In other words, is it possible (at present or in future vaccines) for the safe variation of the virus (or whatever the delivery agent) to be passed from person to person?

I understand that without symptoms like coughing or sneezing, the vector for spreading would be relatively limited, but I have to imagine it's still possible (even commonplace) for healthy individuals to effectively spread bacteria/viruses (even harmless ones).

If it is possible, what are the chances of that happening unintentionally? Would it be ethical to engineer a vaccine to intentionally do that?

I'm not trying to imply that's what's happening now - but I am curious.

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In principal, viral vector vaccines are technically transmissible, though I do not think we have demonstrated actual transmission anywhere. The J&J vaccine is a viral vector vaccine.

The mRNA COVID vaccines are not; with those, the vaccine material is mRNA that your body codes into a protein recognizable to the immune system. The mRNA is both unstable (so it doesn't last long inside the body) and not capable of contagious transmission

There is open research, such as https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5777272/, as to whether making intentionally transmissible vaccines is feasible (or even a good idea)

Not a virologist, but I've heard of viruses in the past that mutated into asymptomatic variants, which are then basically contagious vaccines.

Artificially creating such a thing has been discussed, but I don't think anyone's ready to go there for ethical reasons. What if they screw it up?

Also, you still need _some_ symptoms to get people to sneeze the "vaccine" to their neighbors. It's a very fine line to tread.

Sterilizing immunity is the term you're looking for, and as far as I know the vaccines are still being studied to determine if it happens. It's very difficult to determine right now because you need to very carefully control a studied person AND all of their contacts and people they interact with--you test all of those people daily and then can tell if people in the vaccine or placebo group have close contacts coming down with sickness more or less often (hinting at the presence or not of sterilizing immunity). The big trials for EUA of the major vaccines declined to do this kind of study because it takes longer and costs a LOT more to do that much testing across 60k people + all their contacts.

It will also be more challenging going forward to do these kinds of studies because of the ethics. It will be hard to ethically justify keeping an arm of a trial unvaccinated as a placebo in the face of a very serious and very deadly virus which we have known good vaccines. There are some interesting things like crossover trials that apparently more of the current in research vaccines are trying.

> Could a COVID-19 vaccine (or any vaccine for that matter) have contagious immunity? In other words, is it possible (at present or in future vaccines) for the safe variation of the virus (or whatever the delivery agent) to be passed from person to person?

If a vaccine used a virus that was both live and contagious but somehow safe, its not impossible; of course, there’d also be a risk of it mutating to become unsafe.

The actual vaccines for protection against COVID-19 that I know of don’t use a delivery mechanism where that is possible; that’s certainly not something, e.g., a mRNA vaccine could do.

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