The most common website is USPS by a significant margin. Could that explain much of the international traffic?

COVID tests, I assume.

"Free of charge" pen testing more like. You may have missed the news but since yesterday there aren't even any commercial flights between China and the US anymore, they were all cancelled.

Immigration and Travel too

My guess is this is mostly related to immigration and travel related

Both of these countries have around 1.4B people. 1B people is around 17x Germany, or around 21x the UK.

I think you're missing the .4 of 1.4B in your second sentence.

Uh, yes :)

The US consulates and embassy in China are the only reliable source of information regarding air quality and pollution.

Also, India having a large number of English speakers, probably has a significant percent of the population that prefers getting COVID-19 info from the CDC.

Indeed, as I look now 42% of all traffic is international.

Probably just looking at immigration/Embassies related websites. It is day time in India and China.