Pastor Max Lucado has tested positive for Covid-19 despite having already been fully vaccinated against the virus.

The bestselling Christian author and megachurch pastor posted a tweet showing a photo of a positive coronavirus test, highlighting that the positive result prohibited him from preaching on Sunday at Oak Hills Church in San Antonio, Texas.

Lucado tweeted: "Groan. Covid found me. Tested positive yesterday."

He added, "Was planning to preach today @oakhillschurch and go with some guys on epic golf trip to Ireland tomorrow. Turns out it's me in a downstairs room with aches, stuffy head and quarantine."

However, he also stated that he had much to be thankful for; good medical care, his wife, and that fact that he was at home and not self-isolating in a hotel.

He also posted: "Though miserable, the misery would have been worse with no vaccination. So doing my best to count blessings."

Various reports have indicated that the Delta variant of the Covid-19 virus, which is currently spreading throughout the United States and Europe, may reduce the effectiveness of coronavirus vaccines.

However, even though the efficacy of the vaccines may be slightly reduced, the main vaccines available in the West do appear to remain extremely effective at preventing hospitalisation, and severe cases of Covid-19.

Advisory Board, a healthcare resource group has commented: "Researchers have estimated the [Pfizer] vaccine was just 39% effective at preventing infection in Israel in late June and early July, compared to 95% from January to early April. However, the vaccine was more than 90% effective at preventing severe Covid-19 in both time periods."

It was reported last week that there have been around 65,000 cases of "breakthrough infections" in 27 states in the U.S. - this is when fully vaccinated individuals have gone on to test positive for Covid-19.

The U.S. Centers for Disease Control and Prevention has also reported that it has found over 5,900 cases of fully vaccinated people being hospitalised or dying from COVID-19 as of July 19.  However, this represents a very small percentage of the more than 160 million Americans who have been fully vaccinated to date.