Yates, who won the three-week race back in 2018, was sitting fifth on general classification after Tuesday's individual time trial having finished the stage in seventh place. He was the leader of Team BikeExchange - Jayco, who will now have to adapt their race strategy given the loss of the Englishman.
In a statement team director Dani Castillo said: "During the night our rider Simon Yates had some COVID-19 symptoms with a light fever and body aches.
"We carried out a COVID-19 test this morning and unfortunately, he has returned a positive test result, and will therefore not be taking to the start line of today’s 11th stage, with rider health and wellbeing our priority.”
Sivakov found himself seventh overall heading into Wednesday's 11th 191.2 kilometre stage to Cabo de Gata. His INEOS Grenadiers team have two other riders in the top 10 - Carlos Rodriguez (21) and Tao Geohegen Hart (27) - who will now have one less domestique that can help them in this Vuelta. Ethan Hayter (23), another rider from the INEOS squad pulled out yesterday with COVID, meaning the team is down to six.
The tally of riders who have now been withdrawn from the race with COVID is up to 21 with others also abandoning through undisclosed illnesses. The race still has 10 stages left and it is likely that this number will continue to grow. In this year's other two grand tours there was 19 positive cases.
Remco Evenepoel (22) grew his lead on Tuesday's time trial, winning the stage and now leads the race by two minutes and 41 seconds from Primoz Roglic (32).