 Ships from ports free of plague or completing their isolation period were given a certificate of health and allowed to travel on.A second inspection line was established between the forts on opposite banks of the Thames at Tilbury and Gravesend with instructions only to pass ships with a certificate.The isolation period was increased to forty days – a "quarantine" – in May 1664 as the continental plague worsened, and the areas subject to quarantine changed with the news of the spread of plague to include all of Holland, Zeeland and Friesland (all regions of the Dutch Republic); restrictions on Hamburg were removed in November.