Poland and Sweden are the only EU countries to have not yet passed their COVID-19 peak, the European Centre for Disease Prevention and Control (ECDC) flagged on Thursday.
"The initial wave of transmission has passed its peak in all countries apart from Poland and Sweden," the ECDC wrote in its latest rapid risk assessment.
The EU health agency said that lockdown measures across the EU/EEA — which also includes Iceland, Liechtenstein and Norway — and the UK have led to an 80% decline in the virus's 14-day incidence since the peak on April 9.
But in Poland and Sweden, the 14-day incidence "was at the highest level yet observed".
Twenty-eight of the countries monitored by the ECDC, including Poland, have a current 14-day incidence rate below 20 cases per 100,000 population, while the UK and Portugal have rates between 20 and 100 per 100,000 population.
Sweden was the only country whose rate is above 100 cases per 100,000 inhabitants.
"Increases in testing in Sweden may partly explain this increase," the ECDC said.
Swedish health authorities have recorded 48,288 confirmed cases of the disease as of June 11 as well as 4,814 fatalities. Nearly two-thirds of the country's infections — 29,811 — were reported after April 23, accounting for 6% of the total number of cases observed in the 31 countries since then.
Only the UK, Italy, Spain, and France — Europe's most heavily impacted countries — as well as Germany, had higher case tallies.
A rapid acceleration in the number of cases was also observed in recent days in Poland after clusters were reported at coal mines.
The eastern European nation has reported 28,201 confirmed infections, with more than 1,150 recorded last weekend alone, at least half of which were detected among coal mine workers.
Most EU countries have started to lift lockdown measures imposed in mid-March to curb the spread of the deadly virus although large gatherings and travel within and outside the bloc remain severely restricted.
Several EU nations have already reopened their borders to neighbouring countries with similar rates of infections and a wider reopening is expected on June 15.
Warsaw, which postponed its presidential election because of the pandemic, started to lift lockdown restrictions in mid-May and most businesses including bars and restaurants have reopened. The country also announced on Wednesday that it would reopen its borders to other EU member states on June 13.
Only random checks will be conducted, "exactly as it was before the coronavirus pandemic," the Prime Minister's Office said in a statement.
"Travelers will regain the right to free entry, exit and transit through the territory of the territory of Poland. They will not be quarantined," the statement added.
Sweden bucked the European trend by not imposing a strict lockdown, calling on the population to voluntarily take social distancing measures — a strategy that led to comdenmation from neighbouring countries.
The ECDC noted, however, that "Google mobility data showed that in Stockholm County during the week of 6 April 2020, there was a 49% reduction in the flow of people through transport hubs; a 48% reduction in the number of people working in their usual workplace and a 30% reduction in presence at retail and leisure spaces, compared with baseline levels".