In a study published in the Canadian Medical Association Journal entitled "Impact of population mixing between vaccinated and unvaccinated subpopulations on infectious disease dynamics: implications for SARS-CoV-2 transmission" David N. Fisman of the University of Western Ontario, who earned a Masters of Public Health Degree from Harvard, https://archive.ph/vFzTP claims to demonstrate that the inclusion of an unvaccinated person in a group of vaccinated people increases the risk of infection for those who are vaccinated.
Forbes and other major outlets quickly reported on the findings of the study: https://archive.ph/SHL6e https://archive.ph/nXvp5 https://archive.ph/4A9YD https://archive.ph/6mU8Q https://archive.ph/22Kf1
It is important to understand that the findings in this study are derived from a mathematical model rather than from human participants in real life situations. The use of modeling data to support this type of claim can be problematic if there can be an argument over the extent to which the model is able to approximate real world data. Conclusions reached by a model can be true based on it's assumptions, and it can also be true that the assumptions of the model do not match the factors that determine relevant outcomes in real world environments.
The model used in this study is called the Simple 2-Patch Vaccine Model https://archive.ph/FOSRE and can be found at the following link: https://figshare.com/ndownloader/files/34726612.
In using this model the author acknowledges the work of Garnett and Anderson, "Sexually transmitted diseases and sexual behavior: insights from mathematical models" that provides the approach used in the current study's Balanced mixing between groups. This 1996 article focused on transmission of STDs and the modeling of sexual activity: https://academic.oup.com/jid/article-pdf/174/Supplement_2/S150/2630909/174-Supplement_2-S150.pdf
Other limitations of the study include not accounting for waning immunity https://archive.ph/yc2tt#selection-2079.737-2079.770 number of doses taken (2, 3 or 4) or amount of time since last dose. These are confirmed and relevant real world variables that a complex model should aim to take into account. It is possible that if these factors were included in the model that the findings could be different.
The Competing interests section of the study states that David Fisman has served on advisory boards related to influenza and SARS-CoV-2 vaccines for Seqirus, Pfizer, AstraZeneca and Sanofi-Pasteur https://archive.ph/yc2tt#selection-2537.0-2539.149.
After publishing the study David Fisman went to twitter to promote policies around vaccination mandates and restrictions of freedom for unvaccinated individuals https://archive.ph/5GIjs#selection-639.0-660.0
Vardit Ravitsky also from Harvard https://archive.ph/WshEp made a claim in reference to this study in an interview saying that if a person chooses to remain unvaccinated they significantly increase the risk of infection to those who are vaccinated.
This claim was also repeated on twitter https://archive.ph/VnRaC#selection-1431.0-1431.133
Vardit Ravitsky serves as the Chair of the COVID-19 Impact Committee of the Montreal-based Pierre Elliott Trudeau Foundation and therefore may not be inclined to criticize government policy https://archive.ph/0m2tj#selection-1059.327-1071.367.
Vardit Ravitsky also mentions the hacking of DNA and implanted medical chips in this interview:
The argument that a satisfactory level of population immunity can only be achieved when a large majority of the population is sufficiently up-to-date with their vaccinations fails to account for the contribution of immunity gained through recovery from infection and also the pace of waning of immunity from continued vaccinations. Unless these factors are included in the model it would be premature to make definitive claims about transmission risk between vaccinated and unvaccinated populations.