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In latest news, it is reported that:

if vaccinated people get infected anyway, they have as much virus in their bodies as unvaccinated people. That means they're as likely to infect someone else as unvaccinated people who get infected.

But vaccinated people are safer, the document indicates. "Vaccines prevent more than 90% of severe disease"

I've understood in general viral load is related to severity of the sickness. Do we know why would it not be in this case?

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I believe the news report you cited is not accurate in its reporting. Of course, it's not actual science, and we will wait for the release of the actual science, but here's my understanding (which may end up being wrong):

if vaccinated people get infected anyway, they have as much virus in their bodies as unvaccinated people.

This seems to be incorrect. My understanding is that the CDC info looks at vaccinated people who were infected AND developed COVID symptoms, not all vaccinated and infected people. Also, the tests did not look at the virus levels in the whole body, just in the nose and throat. Obviously, the nose and throat release virus into breath which is exhaled, so that directly impacts how contagious a person might be. However, what should be important for the person's own level of illness is the amount of virus in the lungs and other internal systems, not the nose and throat.

This mis-reporting would seem to account for the apparent discrepancy you noted.

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    $\begingroup$ I'd say this is consistent with my understanding, too: there's a sampling bias here. I think the reporter misunderstood and should have written "if vaccinated people get infected anyway and are symptomatic, they can have as much virus in their bodies as some infected unvaccinated people". Those nuances don't always make it to the news articles. $\endgroup$
    – Bryan Krause ♦
    Jul 30, 2021 at 22:34
  • $\begingroup$ I now found these links from CDC: 1 and 2, which claim "Real-time RT-PCR Ct values in specimens from 127 fully vaccinated patients (median = 22.77) were similar to those among 84 patients who were unvaccinated, not fully vaccinated, or whose vaccination status was unknown (median = 21.54)." - so not sure if discrepancy is fault of the news report. $\endgroup$
    – eis
    Jul 31, 2021 at 5:24
  • $\begingroup$ @eis The data in your MMWR link seems consistent with my and Bryan's description. In particular, the MMWR did not discuss "if vaccinated people get infected anyway", but rather (mostly) symptomatic people who were then discovered to have been vaccinated. That is, the group studied was selected from among those who were already known to be sick, not a random sample from among vaccinated individuals in general. $\endgroup$
    – Armand
    Jul 31, 2021 at 12:46
  • $\begingroup$ @Armand however, I think "some" are not consistent - it would seem from the data that pretty much all examined people did have roughly as much virus (in their throats and noses). Of course, the amount of people involved in this study is small and there is selection bias, but at least in this study the levels seemed to be similar in all samples they had, if I'm reading this correctly. $\endgroup$
    – eis
    Aug 3, 2021 at 9:49
  • $\begingroup$ @eis You may have misinterpreted the whisker plots in Fig. 2 from CDC. The larger an RT-PCR Ct value, the less target was present. Normally, Ct values as small as 38-40 are considered very weak/negative results. The plot shows that some of the vaccinated infected had Ct values this small, while none of the unvaccinated group did. $\endgroup$
    – Armand
    Aug 3, 2021 at 12:41
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First of all there is one error in the article: Vaccinated people do not get infected anyways. Vaccination prevents somewhere between 80-90% of infections (see reference 1). There is no perfect protection against infection, but in the symptomatic cases the viral load goes down much faster in the vaccinated people than in the unvaccinated (reference 2):

enter image description here

The figure show the Ct-values of the PCR from infected patients (figure 1 from reference 2). The lower the Ct value, the higher the viral load. However, this is only a test for the presence of viral RNA, it does not state how infectious these patients are. There are good indications (see reference 3) that vaccinated patients are not as infectious as unvaccinated patients.

And this might be the reason why vaccinated people have less severe disease when they get infected. Lower viral loads (or a faster reduction), less infectious virus which both means less cells in the body can be infected and affected. To sum up: Yes, vaccinations are highly recommendable as well are boosters.

References

  1. Impact and effectiveness of mRNA BNT162b2 vaccine against SARS-CoV-2 infections and COVID-19 cases, hospitalisations, and deaths following a nationwide vaccination campaign in Israel: an observational study using national surveillance data
  2. Virological and serological kinetics of SARS-CoV-2 Delta variant vaccine-breakthrough infections: a multi-center cohort study
  3. Longitudinal analysis of SARS-CoV-2 vaccine breakthrough infections reveal limited infectious virus shedding and restricted tissue distribution
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