According to researchers who were conducting Britain’s COVID-19 vaccine trials, saline injections don’t cause a sore arm. That the saline injection doesn’t produce this specific side effect is problematic and could lead to compromised results when it comes to producing another possible COVID-19 vaccine. Someone participating in the trial might pick up on the lack of sore arm. This might tell them what group they’re in, which could then alter their thinking, their actions, and the overall results of the trial. Such an outcome would be adverse and detrimental to the goal of producing another effective COVID-19 vaccine.
Since placebo saline injections don’t produce the same side effects as the actual vaccine, some researchers used a different placebo. At the University of Oxford, researches used a meningitis and septicemia vaccine for a placebo. This vaccine created similar side effects, making it less likely that the subject would have figured out that they’re in the placebo group.
As for other side effects, these are likely to vary significantly. They’ll depend on the person, their body, and their health. Indeed, people who have participated in the various vaccine trials—and don’t know for sure whether they received the vaccine, a saline placebo, or an alternate placebo—have reported body aches, headaches, fever, general discomfort, and exhaustion.