Nope. There's data indicating we must have the second shot for T-cell response, or the vaccine will fail against mutations:
https://www.researchsquare.com/article/rs-226857/v1 https://twitter.com/dvir_a/status/1359603522581454856 (done on the Pfizer vaccine, but it's very similar to Moderna)
However, I found it interesting that the vaccine seems to possibly even work somewhat against SARS-1 and MERS, which inspires a lot of hope that the vaccines could be at least partially protective against future variants.
Good job, Pfizer (and Moderna)!
However, current results for a Pfizer 1st-only regiment aren't very encouraging IMHO. It's possible the first dose starts to be really effective exactly at the time Pfizer suggested to have the second dose, and that this increase is rather sharp - But it would be some coincidence, and antibody tests (showing not so good efficiency against variants following 1st dose, and also much larger antibody count after 2nd dose) are contraindicative.
Ultimately, someone will have to do real world analysis and get conclusions from that. Right now, IMHO, there's better reason to think that a smaller Moderna dose is fine than to trust a single shot Moderna vaccine regiment.
If I’m not mistaken the claims that the levels are low (and efficacy only 50%) were because some sources took average of days 1-21 instead of 14-21.
The efficacy claims were because disease levels took more time to drop off than what one would expect following lockdown+start date of 1st shot vaccination if 1st shot were effective. I lack the qualifications to evaluate these claims, but I can tell you all Israeli HMOs prefer the 2 shots regimen.
[0] https://twitter.com/ArielMunitz/status/1356683799568990216
Immunity to cold viruses is so prevalent, there is no chance the effect is pronounced, by now. It has been researched tho, so you may find some studies discussing the issue.
On the other hand, the cross-immunization to SARS1 and MERS is not surprising, since the mRNA vaccines were developed for these and modified for SARS2. (It is not a completely novel vaccine and "rapid development" was only possible, because a lot of basic research was already done on SARS1/MERS. Would have taken a lot longer to identify a molecular target otherwise.)
Actually...according to some studies it is.
https://www.nih.gov/news-events/nih-research-matters/immune-...
https://github.com/dviraran/covid_analyses
"However, an underlying assumption here is that the incidence rates of those that were vaccinated early are similar to the general population. Previous analyses have shown that this is not the case as older populations have lower incidence and lower socio-economic groups have higher incidence.4 Therefore, we perform a sensitivity analysis by adjusting incidence rates using different levels of beta values (Figure 1)..."