Here is an article from the CDC about immunocompromised people getting third doses--
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/recommendations/immuno.html. And this one is from the FDA--
https://www.fda.gov/news-events/press-announcements/coronavirus-covid-19-update-fda-authorizes-additional-vaccine-dose-certain-immunocompromised.
In my case, my CLL is a blood cancer, as are all forms of leukemia and most lymphoma cancer. In CLL, most of us have received targeted drugs that attack "mutant" B-cells. B-cells have an important role in searching out and killing "bad" bacteria and viruses. The problem is that B-cells are destroyed by most chemotherapy drugs and a lot of other modern anti-cancer drugs. At one point, I had very few normal B-cells and took two drugs for over a year that got rid of the mutant cells and allowed the normal B-cells to replace them. My blood numbers have been good for the past couple of years, including the last few months of treatment, however, it is not easy to measure how good an immune system is working because it is more complex than just B-cells. In addition, even though I am in remission and getting blood work done every 6 months, CLL is a chronic disease, meaning it is expected to come back some time, hopefully a lot of years from now! The purpose of the research study I participated in was to see how long it would take to come back after the specific two-drug combination I received. Plus, the version of CLL I had was one of the more aggressive forms of the disease because of the specific genetic changes that were found in my blood. (I am unmutated with no gene deletions.)
I am part of a CLL patient forum and found out about the problems with the COVID vaccine not working well for fellow patients way back in early May. You can imagine the frustration of patients who got both vaccines and thought they were safe without masks and then found out they might have to wear masks and be super-careful forever!! At the time, a third dose was not recommended because it was thought if two doses had not worked, another one was likely not to work either. This was very frustrating for some patients who went through the two doses only to find that their immune systems were still not working. They found this out by getting something called a "SARS Spike Protein IgG Antibody Test." I had the test in early June, and it showed a reaction to the vaccine, but not the level of the reaction. (Others did find out level of reaction, but mine just showed a reaction, without detail.)
In the meantime, there has been some research with immunocompromised patients that shows that the spike protein level of most patients DOES improve with a third dose, which is why the CDC announced their recommendation a few days ago. That and recent announcements that the levels of effectiveness of the vaccines does go down over time, which is why the 8-month time period for most people. However, like everything else about COVID, a lot is not known, so I and a bunch of other people got or are getting a third "booster" dose after 5-6 months, per the CDC and FDA recommendations.
It may be that I and other cancer survivors are overdoing it, but then again, maybe not, especially with the new Delta virus. In any case, I wanted to make sure I stayed healthy and could be fairly confident that the vaccines are working for me personally.
Hope this explains why some of us were able to go to the head of the line, so to speak, for third doses.