An alarming drop in COVID-19 testing threatens to undermine India’s efforts to contain the pandemic, the federal health ministry said in a letter to state governments on Wednesday, as concerns about new waves of infection abroad grew.

On Wednesday, India reported 9,283 new COVID-19 cases, a day after recording 7,579 infections – the lowest in 543 days. However, testing has been hovering around 1 million per day for the past few weeks, which is less than half the capacity.

“It is very difficult to determine the true level of infection spread in a geography in the absence of sustained levels of sufficient testing,” Health Secretary Rajesh Bhushan wrote in a letter to Nagaland, a northeastern state that has reduced testing by more than a third in two months.

“With the majority of countries experiencing multiple surges in COVID cases in recent times, and a few developed countries experiencing fourth and fifth waves despite high levels of COVID vaccination, there is a need for continued vigilance given the disease’s unpredictable and contagious nature.”

The ministry delivered the same warning to other states, though each letter contained observations specific to the various states’ circumstances.

India has so far reported 34.5 million infections. The United States has the only higher total number of cases. Deaths in India rose by 437 on Wednesday to 466,584.

Health workers have started a door-to-door vaccination campaign in India as tens of millions of Indians have not taken their second dose by the due date.

India has so far administered 1.18 billion vaccine doses – at least one dose in 82% of its 944 million adults and two in 44%.