"Because this was a new strain, VESKs antivirus provider Sophos had not yet been updated to detect it - something other antivirus providers were also yet to do."
Antivirus was great against viruses. But that's not the threat now. The threat is malware sent by email, or in malvertising. If you take one of the malware-carrying emails (I get hundreds each week) and show it to virustotal.com, then typically the malware might be detected by 10% of products.
You should not rely on antivirus to protect you from non-virus types of malware.