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My interest was inspired by the observed variation in incubation times for different strains of Covid-19, however I ask the question in the broader sense as it seems hard to find an answer in general.

Does the presence of immunity affect the duration of incubation period? I would have thought the body's ready ability to identify and defend against a virus may mean that it is identified sooner following infection. But I can't find evidence to confirm or refute this idea. Would really appreciate any thoughts!

Thanks

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    $\begingroup$ Sorry - I'm not sure on the question here - once a virus enters a cell it will replicate at approx the same rate no matter whether there is an immune response. Immune response could prevent infection/limit infection, but won't affect replication kinetics. $\endgroup$
    – bob1
    Commented Dec 31, 2021 at 7:54
  • $\begingroup$ I feel contradiction: if there is "incubation" there is bound to be "no immunity". $\endgroup$
    – Peter Bernhard
    Commented Dec 31, 2021 at 19:47
  • $\begingroup$ @bob1 It was a question of what is really happening during incubation. During incubation the virus is replicating undetected, but at some point it becomes detected and is no longer in the incubation phase. So I'm curious whether having a better immunity to a particular virus would result in a quicker detection in this phase, therefore shortening the incubation phase. Thanks for your answer $\endgroup$
    – Harry_Newton
    Commented Jan 2, 2022 at 2:01
  • $\begingroup$ @PeterBernhard that's not true at all. The evidence is all around you - people who have been vaccinated are still getting sick with the omicron variant. Granted, not as sick as with other well-publicized strains, but sick nonetheless. The same thing happens with delta, to a lesser extent. So clearly, people with (good) immunity can still have virus incubation and eventually illness. $\endgroup$
    – MattDMo
    Commented Jan 3, 2022 at 13:00
  • $\begingroup$ @MattDMo Thanks to your comment I now feel better ... with the question that seems to have incubated on me, just as your comment seems to have. I will delete that expression of my feelings up above, as it hurts. Sure, me too, I have been misunderstood: it's in your parenthesis, what I was up to, i.e. how can you use the word well - immunity - if incubation is evidence for its failure which is non existence. That is a debate on words and definition. Maybe more, though. (What is: symptomless infection) I feel understood. Still, I will delete my comment. I always did agree with all you say. $\endgroup$
    – Peter Bernhard
    Commented Jan 3, 2022 at 17:54

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There are two effects that can shorten the observed incubation period in presence of prior partial immunity

  1. Symptoms are caused mainly by the reaction of the body, not by the virus itself. When there is some prior immunity, the immune response kicks in faster, leading to an earlier onset of symptoms. This effect is genuine.
  2. When the infection proceeds somewhat slowly, it may be completely terminated by the immune response without any symptoms occurring. This effect cuts off the long tail of slow infections, leading to a shorter average incubation period.

EDIT: Not a publication but a Twitter thread by a renowned scientist (Micheal Mina) confirming shortening of incubation period by immunity.

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