Please refer to this article:

https://www.ceps.eu/measuring-price-stability-in-covid-times/

The article discusses the use of alternative measures of inflation for macroeconomic policy response to the Covid pandemic in the European Monetary Union (EMU). The article compares three measures of inflation: GDP deflator, the harmonized index of consumer prices (HICP), and the core, which is calculated the same way as HICP, but excludes energy and food items.

The article claims that since the outbreak of Covid-19, HICP may be understating the actual inflation rate. What is the article's reasoning for this claim?