Side effects will typically be a greater possibility with a two injection vaccine like the mRNA vaccines being used for COVID. The first dose basically introduces an inactive virus into our bodies which begins to the creation of neutralizing antibodies. Then, along comes the second dose and this spurs a greater reaction because now the immune system has been alerted, and that triggers an immune response. Some people who have a mild reaction to the first dose may have already been previously infected with COVID, but were asymptomatic and may never have known they were COVID positive. My personal vaccinations were back in February (1st and 22nd) with the Pfizer/BionTech vaccine and basically nothing unusual after the first shot, but after the second dose I got pretty tired later that evening (fatigued, I suppose), but nine hours of deep sleep rectified that. The next day I had some mild sinus headaches and some nasal congestion, but a single Zyrtec handles this and by the evening I was back to normal. Side effects are typically more intense in younger people (say under 55) because their immune systems are more robust, where as in the elderly our immune systems slowly, gradually diminish.

BTW, The Johnson & Johnson/Janssen vaccine is still a highly effective, state of the art vaccine that should not be passed on if the opportunity arises. The key is to prevent serious respiratory illness and eventual hospitalization, and all of the present COVID-19 vaccines deliver in that area. And, despite what might be on the news you can safely travel 13 to 15 days after the second dose (or first dose with J&J) if you want to travel and deal with the hassle of all the regulations in place.