These are pretty common, physiologic structures associated with infections. They can be just a handful of cells on a slide or be quite large, and I don't know what they found in these infections. I didn't read the original paper. The ectopic lymphoid structures go away after the infection resolves. It seems that the immune system has ways to set up mini lymph node architecture right by the site of infections, which is very sensible. The same process is going on in a more organized way in the draining lymph node in parallel. Research into these was really hot in the 2010s, but people don't seem to be as into them anymore (but my research has also transitioned to innate immunity from adaptive, so it's likely that I'm no longer in that universe).
In general, it doesn't surprise me that when you prime the innate immune system, the adaptive immune system works well. The problem is that pathogens have an incredible suite of tools ready to evade these mechanisms. The doses of the pathogens are typically insanely high too, which I do not think model natural infections well. Anyways, this is intriguing, so I'll take a look at the original paper one of these days. Vaccine research generally is so boring. It's like, we vaccinated, and it worked, or didn't, no mechanism.