WELLINGTON, New Zealand--Infant formula firm a2 Milk Company Ltd. lowered its full-year earnings forecast for a third time since September as the pandemic continued to slow sales via Chinese surrogate shoppers.

Shares of the New Zealand- and Australia-listed company plunged by more than 17% in NZX trading on Thursday, wiping 1.7 billion New Zealand dollars ($1.26 billion) from its market value. The shares are down about 57% since August.

The company forecast a full-year profit margin of between 24% and 26% compared with a forecast of 26% to 29% in December.

It said full-year revenue would be "in the order" of NZ$1.4 billion compared with a range of NZ$1.4 billion to NZ$1.55 billion previously.

The pace of recovery has been "slower than previously anticipated," a2 Milk said. Its new forecasts assume that quarterly growth improves significantly from the third quarter to the fourth quarter.

The company's first-half profit of NZ$120 million was 35% lower compared with a year earlier.

Write to Stephen Wright at stephen.wright@wsj.com

(END) Dow Jones Newswires

02-24-21 1725ET