The peoples health institute (FHI) of Norway came out yesterday and said that 25% of they who tested positive with CoVid19 in Norway the last 2 weeks caught it abroad.
the main things are what is the status of the virus transmission in the country coming from or going to, if low prevalence and under control such as NZ to Singapore risk is lower than say someone from the UK where it would be very high risk
Station Zero has hit the nut on the head with the second quote. I have repeatedly said that failing to close borders in Q1 2020 led to the situation we now face and was the biggest failure. Over the summer we also needed to control borders and PCR swabs taken at day 4 onwards can never do more than reduce sensible quarantine to 8 days, but now the UK is at the top of the league and the same as much of europe and the US it makes little difference whether you control the borders or not. Another paper on monday showed no difference at all in infection rates between those that had been abroad and those that had not. Norway is a little different.
So epidemiologically we could simply open the borders and allow free travel while the rate of infection is similar between countries. I see you do not have to quarantine coming to the UK from New Zealand for example, but going the other way is a different story