According to recently filed accounts for the restaurant chain’s parent company Oaxaca, loss before tax for the year to June 2017 was £4.7m, compared to a profit of £640,000 the previous year.
Revenue was almost flat at £46.6m, but the cost of sales rose to almost £28m.
The Mexican restaurant brand was forced to close nine of its London sites in October and November 2016 after an outbreak of norovirus that saw 200 staff and 160 diners fall ill.
In documents filed at Companies House Wahaca co-founder Mark Selby described the incident as a “one-off event” that resulted in exceptional costs of £700,000.
He said: “The results show the impact of a one-off event in October 2016 whereby an outbreak of Norovirus, widely reported in the media at the time, caused us to voluntarily close a number of our restaurants on a short-term basis, with a consequential loss in revenue and profit.
“Whilst this had a significant impact on our trading performance for the financial year, relative to the previous year the business has recovered from the event and continues to be in a strong position to grow, with ambitious future plans.”