> the UK has better infant mortality than the state with the best infant mortality, Massachusetts
The US uses a different definition for infant mortality than the UK or any other European country. (One major difference is whether a borderline-nonviable birth gets counted in the infant mortality or stillbirth column.) The raw numbers are not directly comparable. Correcting for the expected effect of difference in definitions, the UK still has lower infant mortality than the US overall, but by a far smaller factor.
You are talking about a difference which would reduce recorded US infant mortality by around 15%, when it's nearly double that of Western European countries, and infant mortality in poor regions and for ethnic minorities is far more than that.
The US uses a different definition for infant mortality than the UK or any other European country. (One major difference is whether a borderline-nonviable birth gets counted in the infant mortality or stillbirth column.) The raw numbers are not directly comparable. Correcting for the expected effect of difference in definitions, the UK still has lower infant mortality than the US overall, but by a far smaller factor.