For four hours on October 13, the Covid-19 pass system used by England’s National Health System (NHS) went offline, stranding travelers at airports around the country who needed to show digital proof of their vaccinations to board their flights. The NHS acknowledged the outage, but not the cause, tweeting, “There are currently issues with accessing the Covid Pass on the NHS App and website.” The offline pass system caused chaos and delays at the gates of many airlines. “No system is bullet-proof, as we saw a few weeks ago with the Facebook/WhatsApp/Instagram outage,” commented Avast’s Luis Corrons. “That’s why it’s important not to rely exclusively on online services. For example, in this case you are given the option to download the certificate in PDF format, which you can either print out or save on your phone.” The UK is just one of many countries that require vaccine passports in order to travel. For more on this story, see Ars Technica.