Can it be? Study finds that college students who had been vaccinated against flu were more likely to excrete flu virus / Proceedings National Academy of Sciences

It is only one study.  But it was carefully done, used interesting methods, and it tried to explore issues that have rarely been studied.  Researchers from the University of Maryland studied students who had new symptoms of influenza to see if infectious flu virus spread during coughs, sneezing, or regular breathing.  They looked at nasal swabs as well as exhaled air during normal breathing. 

They found that sneezing is rare during flu and is not important in spread of flu.  They found that cough is not necessary to spread flu:  simply breathing excretes plenty of influenza virus.

This is important because fine aerosols generated simply by breathing remain suspended in air for relatively long periods.  Current thinking is that infectious particles larger than 5 microns fall to the ground quickly, but particles smaller than 5 microns may remain in air longer, travel further, and be infectious at larger distances from an infected person.  So staying 3 feet or 6 feet away from a person with flu will not be sufficiently protective.  Typical infection control for flu in healthcare facilities usually involves contact and droplet precautions, for spread over short distances only.

However, this study suggests this may be inadequate, if airborne transmission is a major contributor to spread.  Prevention of airborne (fine aerosol) transmission requires special air handling and the use of negative pressure rooms.  Visitors to the rooms of flu patients will be at risk.

Twenty-two of the University of Maryland subjects with influenza had received flu vaccinations during both the current season and the previous season.  Surprisingly, this group had significantly greater shedding of viral RNA in fine aerosols, compared to subjects who had not been vaccinated in the current or prior season.  This raised the question whether vaccination might actually increase the spread of influenza.  Hopefully other research groups will pay attention to this finding and help confirm or disprove it.