In puerto rico we have 80% of the population vaccinated and the positivity is very low like .2% we will have a family reunion , we are all vaccinated except my 5 y/o niece which is attending school is this a risk for anyone? for her or us?

It is a potential risk, since children may have few or no obvious symptoms awhile infected. They may then spread the infection. Those individuals with weakend immune systems, e.g. the elderly are particularly susceptible. The high vaccination rate though does generally offer protection. A mask and good hand washing is recommended.

Even one dose of vaccine will be helpful for the 5 year old, ideally a second dose 3 weeks later. Use as many precautions as you can: handwashing, mask, social distancing. If other kids she will be with can wear masks also, that is helpful. Ultimately she is at risk for acquiring infection and you will want to do everything to minimize that risk.