The Tokyo Stock Exchange, which was on the plus side ahead of the announcement, fell in the closing Friday trading.
At the same time, the dollar is rising, as investors are investing in safe cards in what they experience as times of unrest.
In the commodity market, the oil price is under pressure, while the gold price - which before the announcement was in the red - is turning upwards.
Additional uncertainty
It also points downwards to leading European stock exchanges and the US stock exchanges in Friday's trading in the so-called futures trading.
European stock markets appear to be down just over 1 per cent, while in the US there are declines of around 2 per cent.
The announcement that Trump has covid-19 was made by the president himself via Twitter and comes just over four weeks before the US presidential election on November 3.
Lina Fransson, interest rate strategist at SEB, says that the illness announcement thus entails further uncertainty ahead of an already uncertain autumn.
- We are at a stage where the American election is in the middle of the final sprint, and the signals so far are that it will be a very messy election process.
In addition, we have ongoing spread of infection and Brexit negotiations that contribute to uncertainty, which the financial markets do not like, she says.
- That the incumbent president then announces that he has tested positive for corona will be another factor of uncertainty.
"Instinctive reaction"
Whether it will have a more far-reaching effect on the world's stock exchanges remains to be seen, says Lina Fransson.
- It is hard to say.
This became an instinctive reaction in the market, and then you have to follow the development.
TT: How do you think it affects the Stockholm Stock Exchange?
- You can see that the futures in Europe are also pointing downwards, so most indicate that the Stockholm Stock Exchange is also turning downwards, she says.