Immunity to ordinary respiratory viruses occurs mainly through T-cells which 'take a picture of the invader' at a molecular level, 'reproduce' it on certain immune cells and essentially 'never forget a face'. This T-cell immunity is robust and durable. Those exposed to the highly related SARS virus in 2003 still have this immunity 17 years later. In relation to SARS-CoV-2, the pattern of immunity to date is identical and after around 800 million infections across the world, there is no convincing evidence for significant levels of re-infection. Not only are those who've been infected and have now recovered immune (they cannot get ill again with the same virus), but importantly they do not participate in transmission. (See my article on what SAGE got wrong for Lockdown Sceptics.) Furthermore, because the immune response is diverse, a proportion of them will also be immune to novel but similar viruses in the future.