Tom Cowan - the Horrors' keyboard player - joined his brother Freddie for The Vaccines' spot on the NME/Radio 1 stage, while he later arrived on stage for the Horrors' closing number 'Moving Further Away' in the Festival Republic tent.
The Vaccines performed a number of hits from their debut album What Did You Expect From The Vaccines?, which peaked at number four on the album chart earlier this year.
The band took to the stage despite singer Justin Young suffering with a throat infection. Pete Robertson told Digital Spy: "He's suffering a bit so he's preserving his voice. It's nothing serious though - just a precaution before the performance."
Their 50-minute set included 'If You Wanna', 'All In White', their debut single 'Wreckin' Bar (Ra Ra Ra)' and breakthrough hit 'Post Break-Up Sex'.
The four-piece also performed their latest single 'Nørgaard'. A live performance of the track appears below:
Digital Spy is bringing you news from Reading Festival 2011 all weekend. Click here to read our full coverage of the music event.