December 19

Apologies if this has been asked here hundreds of times already, but why is there a need to vaccinate multiple time for some viruses, such as Covid, whereas other viruses and illnesses can be stopped with one vaccination?95.150.37.233 (talk) 22:28, 19 December 2021 (UTC)[reply]

Some viruses mutate more frequently than others do. --←Baseball Bugs ^{What's up, Doc?} carrots→ 23:27, 19 December 2021 (UTC)[reply]

That's why we have annual vaccinations for flu, but I think the question was about vaccinations such as the Pfizer and Moderna COVID vaccine, which would have required two doses even against the original COVID strain. Now see Lambiam's answer below. --174.95.161.212 (talk) 07:42, 20 December 2021 (UTC)[reply]

Vaccination tricks the immune system into producing antibodies against an infectious agent. The trick is that the vaccine contains stuff that is not by itself pathogenic but resembles the infectious agent. (With mRNA vaccines, there is a double trick: the vaccine does not contain the mimicking stuff but contains a recipe to make the body of the recipient produce it.) Now, if you are lucky, the resulting antibodies complete neutralize the infectious agent. The main reason is then usually that the infectious agent has a single weak spot: once an antibody binds to it, it can no longer enter cells. In that case, the vaccine is said to confer "sterilizing immunity" (see Vaccination § Preventing disease versus preventing infection). The mRNA vaccines against SARS-CoV-2 result in antibodies that bind to a so-called "spike", which the virus needs to gain entrance to a cell. SARS-CoV-2 is a coronavirus, and these have an awful lot of such spikes. To completely neutralize a coronavirus particle, antibodies need to neutralize each and every of its spikes. That is unlikely to happen after a single or even two vaccinations. But even if not all spikes are neutralized, neutralizing many spikes slows down the speed with which the virus replicates, buying the immune system time to produce sufficient antibodies to vanquish the invader. It was known from the start that the vaccines that were going to be developed against the COVID-19 pandemic would not confer 100% immunity. But, by a regime of two shots with sufficient time between them, the second shot makes the immune system, already on alert to the invader, produce significantly more antibodies than a single shot with a double dose can do.  --Lambiam 02:19, 20 December 2021 (UTC)[reply]

Nitpick: live vaccines contain viable pathogen, but "weakened" so as to usually not produce a "serious" infection. Though I don't believe any live COVID-19 vaccines are in non-experimental use. --47.155.96.47 (talk) 02:34, 23 December 2021 (UTC)[reply]

• For the general case, I would add to the above that immunology in general is complicated, and vaccine-making is full of tricks. For instance, we do not have a vaccine against Clostridium tetani (the bacterial agent), but we still have a vaccine against tetanus (the disease) that works by setting off the immune system to neutralize the toxin that causes the big symptoms.

For the very specific case of COVID-19, there are actually both double-dose and single-dose vaccines available. See COVID-19_vaccine#Vaccine_types for details, but to name the "big four" where I live (France), Moderna/Pfizer are double-dose mRNA vaccines, Janssen is single-dose viral vector, and AstraZeneca is double-dose viral vector.

I am actually somewhat surprised that AZ uses the same content for both doses, while I would naively expect that viral-vector is a one-time trick (the second time around, shouldn’t the immune system fight the carrier instead of the payload?). Sputnik uses different carriers for both doses.

Plausibly^{[original research?]}, the RNA stuff would work decently with single-dose, but the manufacturing companies decided to launch the approval studies with double-dose right away - it is much better to overshoot the efficiency thresholds than to undershoot. Tigraan^{Click here for my talk page ("private" contact)} 10:21, 20 December 2021 (UTC)[reply]

We know from published studies that the protection after a single-dose mRNA vaccination is considerably less than after the second dose.^[1][2] I think this was to be expected.  --Lambiam 13:00, 20 December 2021 (UTC)[reply]

Of course, but even so, the vaccine efficiency (≈ decrease in probability of being infected) are still quite high, 70%+ in your first source. It would still meet the FDA guidelines (see here, "statistical considerations" page 14) of 50% observed efficiency / 30% estimated lower bound efficiency. By way of comparison, the seasonal flu vaccine is around 20-50%. My point is that Pfizer/Moderna could have tried to get approval for a single-dose schedule, but did not, because the positives (faster phase III trials) were minimal compared to the risk of catastrophe (not meeting the approval threshold). Hence, whether a vaccine is single-dose or double-dose is dependent on business considerations as much as medical considerations. Tigraan^{Click here for my talk page ("private" contact)} 14:50, 20 December 2021 (UTC)[reply]

Existing immunity to the viral vector would be an issue if it meant sterilizing immunity prevented the vector from infecting cells and getting them to produce the antigen "payload" sufficiently to trigger an immune response to it. I'm not sure how much of an immune response gets triggered against the vector. I suspect there's a lot of variation. Not all viruses produce a strong immune response. --47.155.96.47 (talk) 02:34, 23 December 2021 (UTC)[reply]

Immunity tends to naturally "decline" over time. Repeated exposure strengthens it. Also, not everyone will necessarily develop sufficient immunity after one dose; this notably is a factor with the MMR vaccine. The majority of general market vaccines either require a multi-dose series, or revaccination if exposure is anticipated and the previous vaccination has "worn off". Examples: tetanus vaccine, cholera vaccine. An interesting case is human rabies vaccines, which are most commonly given as post-exposure prophylaxis (after a suspected exposure to rabies). A suspected exposure always calls for a full vaccine series, regardless of past vaccination, because possible adverse effects are outweighed by the essentially guaranteed death from rabies if existing immunity is not sufficient. --47.155.96.47 (talk) 02:34, 23 December 2021 (UTC)[reply]