It clearly is (partly) because it could cause harm:
"Ms Bingham said vaccination policy would be aimed at those “most at risk” and noted that vaccinating healthy people, who are much less likely to have severe outcomes from Covid-19, “could cause them some freak harm”, potentially tipping the scales in terms of the risk-benefit analysis."
It doesn't look like she's talking about established widespread side effects, and is being cautious. She obviously isn't confident that it's 100% safe but it is safe enough to use.