Thanks, Joe. And good morning, everyone, I appreciate the opportunity to provide some updates on what is going on at ImmuCell.
We have several top priority challenges to meet presently. At the top of the list is increasing our product sales and expanding our production capacity to meet growing demand. I would add obtaining the final regulatory approval of Re-Tain from the FDA to the top of the list of challenges right in front of us. I'm excited to introduce our VP of Manufacturing Operations and our VP of Sales and Marketing, two key people who are being extremely affected in meeting these challenges for us later in this call. I'll let you hear directly from them.
First, I'll do a quick run through the numbers, the press release that we disclosed last night summarizes the un audited financial results. Since you have that information, I will not take your time here to review all the line-item detail, but I would like to review some of the highlights.
As you may know, on 5th, we issued a press release covering our preliminary top line sales results. We were making these optional announcements to give investors a very timely look at what I view as the most critical measure of our operations and financial performance that being product sales early in the reporting period. I have no changes to that very strong previous disclosure. Again, product sales were up 45% during the fourth quarter of 2021, and up 25% for the year ended December 31, '21, in comparison to the respective periods of the prior year. The gross margin as a percentage of sales improved to 47% during the fourth quarter of 2021, compared to 43% during the fourth quarter of 2020.
Our gross margin percentage was 45% during both of the full years ended December 31, '21 and 2020. The higher level of sales and improved gross margin helped us report net income of $74,000 during the fourth quarter of 2021 and helped us reduce our net loss to $78,000 during the year ended December 31, 2021, from a net loss of just over $1 million during the year ended December 31, 2020. Earnings before interest taxes, depreciation, and amortization or EBITDA, increased to $2.7 million during the year ended December 31, 2021, compared to $1.9 million during the year ended December 31, 2020. These non-GAAP financial measures should be considered in context with our statement of cash flows that is presented in accordance with GAAP. By way of an update on the status of Re-Tain, I'm pleased to report that we made our second submission to the FDA to address the FDA's previous comments, inquiries on the chemistry, manufacturing and controls, or CMC technical section, which is required for approval of our new animal drug application or NADA. This complex submission has been a very significant effort by our regulatory, manufacturing, and quality team. In August, we will find out if the FDA either approves our NADA or ask for further questions.
Our product development objective is demonstrated that our peptide and microbial niacin A, can play a productive role in the treatment of subclinical mastitis in today's dairy industry, and offer an effective alternative to traditional antibiotics. Because label requirements of all intramammary drugs on the market today require that milk be discarded and that meat be withheld during treatment and for a period of time thereafter, it is common practice in the dairy industry today not to treat sick cows that are still producing saleable milk. Re-Tain provides an animal welfare benefit by removing this economic disincentive to treating subclinical mastitis, and allowing sick cows to be treated without the milk discard and meat withhold penalties.
In addition to improved animal welfare, Re-Tain enhances food safety and sustainability by utilizing a peptide antimicrobial that is not used in human medicine. This is important because the overuse of traditional antibiotics is thought to create antibiotic resistance, which is an ongoing public health concern. Outside of the development of Re-Tain, probably the most exciting work going on at ImmuCell right now, is the growth in both of our production capacity and sales demand for the First Defense product line. I've asked our VP of manufacturing operations, Betsy Williams and our VP of Sales and Marketing, Bobbi Jo Brockmann to speak about the exciting work. Betsy, let's start with you, please. It was not too long ago that our previous production capacity of about 16.5 million per year was an up to meet demand.
However, as of the end of every quarter since March 31, 2020, we have had a backlog of orders. Would you please speak to the capacity expansion initiatives we've completed over the past two years, that enabled us to increase our output to almost $23 million on an annualized basis during the fourth quarter of 2021.