Infection segmentation on chest CT plays an important role in the quantitative analysis of COVID-19. Developing automatic segmentation tools in a short period with limited labelled images has become an urgent need. Pseudo label-based semi-supervised method is a promising way to leverage unlabelled data to improve segmentation performance. Existing methods usually obtain pseudo labels by first training a network with limited labelled images and then inferring unlabelled images. However, these methods may generate obviously inaccurate labels and degrade the subsequent training process. To address these challenges, in this paper, an active contour regularized semi-supervised learning framework is proposed to automatically segment infections with few labelled images. The active contour regularization is realized by the region-scalable fitting (RSF) model which is embedded to the loss function of the network to regularize and refine the pseudo labels of the unlabelled images. We further design a splitting method to separately optimize the RSF regularization term and the segmentation loss term with convolution-thresholding method (ICTM) and stochastic gradient descent, respectively, which enable fast optimization of each term. Furthermore, we build a statistical atlas to show the infection spatial distribution. Extensive experiments on a small public dataset and a large scale dataset show that the proposed method outperforms state-of-the-art methods with up to 5\% in DSC and NSD, 10\% in RAVD and $8mm$ in 95\% HD. Moreover, we observe that the infections tend to occur at the dorsal subpleural lung and posterior basal segments that are not mentioned in current radiology reports and are meaningful to advance our understanding of COVID-19.