BRUSSELS, Dec 3 (Reuters) - Belgium tightened its coronavirus restrictions for the third consecutive week on Friday to fight one of Europe"s worst spikes of COVID-19 cases, but stopped short of the strict curbs imposed in the neighbouring Netherlands or Austria. "We cannot allow the train of infection that is thundering through our country to continue at its current pace," Prime Minister Alexander De Croo told a news conference.With cases among children rising the most, De Croo said mask mandates will apply from age six. The current requirement to wear a mask is for those aged 10 years and older. Primary schools will shut for Christmas and New Year holidays a week earlier and secondary schools will shift to a hybrid system, with half of classes from home.By contrast, bars and restaurants in Belgium, home to EU institutions and NATO, will still be able to open until 11 p.m, six hours later than in the Netherlands. De Croo acknowledged that Belgium"s infection rate was among the highest in Europe. Only Slovakia and the Czech Republic have higher 14-day average per capita rates, according to European Centre for Disease Prevention and Control data.However, although more than one in 50 people have been infected in the past two weeks, the fatality rate is only just ahead of the EU average. "There is only one reason for this and that is that the vaccination rate in our country is particularly high," De Croo said, adding that around 4 million Belgians should have had booster doses by the end of the year. Experts of health agency Sciensano said Belgium appeared to have hit a peak of infections and hospital admissions, although intensive care patients could rise to 1,000 within a week. "It is not clear whether after this we can expect a sharp decline or if we will just remain on a high plateau," virologist Steven Van Gucht said.