This 2018 Collective Evolution article claims:

Once recent study, however, did bother to look at the question of whether the vaccine prevents transmission. Published on January 18, 2018, in the journal of the Proceedings of the National Academy of Sciences of the United States of America, PNAS, the study’s authors screened volunteers with confirmed cases of influenza and took breath samples. And among their findings was “an association between repeated vaccination and increased viral aerosol generation” [...]

In fact, subjects who had received the influenza vaccine in both the current and the previous season were found to shed over six times more aerosolized virus than those who did not get a flu shot during either season.

Do people who receive flu shots shed the virus more than people who don't?

A grain of truth, but not true in general

The claim however...

In fact, subjects who had received the influenza vaccine in both the current and the previous season were found to shed over six times more aerosolized virus than those who did not get a flu shot during either season.

...is clear enough to examine and the author has sourced it well. The source is this article: Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community

The title (boldface mine)...

"Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community"

...and the following must be noted (boldface mine):

We screened 355 volunteers with acute respiratory illness; the 178 volunteers who met enrollment criteria provided 278 visits for sample collection. We confirmed influenza infection in 156 (88%) of the enrolled participants using qRT-PCR

In other words: this study concerns people that at the time were sick with influenza.

The article then states...

Self-reported vaccination for the current season was associated with a trend (P < 0.10) toward higher viral shedding in fine-aerosol samples; vaccination with both the current and previous year’s seasonal vaccines, however, was significantly associated with greater fine-aerosol shedding in unadjusted and adjusted models (P < 0.01). In adjusted models, we observed 6.3 (95% CI 1.9–21.5) times more aerosol shedding among cases with vaccination in the current and previous season compared with having no vaccination in those two seasons. Vaccination was not associated with coarse-aerosol or NP shedding (P > 0.10). The association of vaccination and shedding was significant for influenza A (P = 0.03) but not for influenza B (P = 0.83) infections (Table S4).

So the authors of the study state that...

• in this particular study


• for people that were currently sick with influenza


• for only one type of influenza


• regarding only fine aerosols


• where the subjects said they had been vaccinated

...they measured increased shedding. But for another type of influenza, for other types of shedding, they did not detect that, even in people that were vaccinated before. Also note that the Confidence Interval is huge. It is not actually 6.3 times more for 100% certain, but they are 95% certain it is somewhere between 1.9 times and 21.5 times more shedding.

So the claim in Collective Evolution has a small grain of truth: in one study they found that for one particular type of influenza, where people had become ill in influenza, and where they had gotten ill despite vaccination, they detected a two-to-twentytwo(ish)-fold increased of shedding in fine aerosols, but not for other types of pathways for shedding. And for the other type of influenza they did not find an increase in fine aerosol shedding.

In the "discussion" section, the authors of the study therefore say the following (boldface added by me):

The association of current and prior year vaccination with increased shedding of influenza A might lead one to speculate that certain types of prior immunity promote lung inflammation, airway closure, and aerosol generation. This first observation of the phenomenon needs confirmation. If confirmed, this observation, together with recent literature suggesting reduced protection with annual vaccination, would have implications for influenza vaccination recommendations and policies.

So the claim in Collective Evolution that it has been proven that for all influensas, all types of aerosol shedding increase if the subject is vaccinated, no matter if they got ill or not, is not supported by this study. The author has ripped out a small section of the original study and extrapolated in ways that are not supported by the study.

Summary

The claim has a grain of truth, in that one study has observed this effect, for people that were vaccinated and still got ill, and only once. But the general claim — that for all influenzas, all vaccinated people increase their virus shedding six-fold — has not been proven. And it is not shown that such shedding warrants a change in policy regarding vaccination.

Significant xkcd: