As you can see in the graph below, after a steady decline in November, confirmed cases in prisons began to rise again throughout the middle of December, reaching a peak in the week ending the 21st, when over 800 new cases were recorded. Cases then appear to have fallen abruptly in the following two weeks.
2,407 confirmed cases were recorded between 1 December and 4 January. That's over two-fifths of the nearly 6,000 cases recorded in prisons since the pandemic began.