(CNN) India, home to the world's worst ongoing coronavirus outbreak, has reported more than 17.6 million cases since the pandemic began last year.
But the real number, experts fear, could be up to 30 times higher -- meaning more than half a billion cases.
Health workers and scientists in India have long warned that Covid-19 infections and related deaths are significantly underreported for several reasons, including poor infrastructure, human error, and low testing levels.
Some things have changed since then -- testing has greatly increased in the wake of the first wave, for instance. But still, the true extent of the second wave now ravaging India is likely much worse than official numbers suggest.
"It's widely known that both the case numbers and the mortality figures are undercounts, they always have been," said Ramanan Laxminarayan, director of the Center for Disease Dynamics, Economics and Policy in New Delhi.
"Last year we estimated that only one in about 30 infections were being caught by testing, so the reported cases are a serious underestimate of true infections," he said. "This time, the mortality figures are probably serious underestimates, and what we're seeing on the ground is many more deaths, than what has been officially reported."
CNN has reached out to the country's health ministry for comment about the claims of underreporting.
As the first wave began to ebb in September last year, the government pointed to its low death rate as a sign of its success in handling the outbreak, and to support its decision to lift some restrictions. Prime Minister Narendra Modi celebrated the low figures as boosting "the confidence of people," and predicted that "the entire country will emerge victorious in the battle against Covid-19," according to a press release in August.
That battle is still ongoing. The country's daily death toll is now projected to continue climbing until mid-May, according to prediction models from the University of Washington's Institute for Health Metrics and Evaluations.
The death toll could peak at more than 13,000 a day -- more than four times the current daily death toll, the predictions show.
"I don't think any family has been spared a Covid death," said Laxminarayan. "There's a missing person in every family that I can think of."
India's testing capacity has increased dramatically since the first wave. Around this time last year, the country was testing fewer than half a million people per day -- now, "they are doing close to 2 million tests a day," said Dr. Soumya Swaminathan, chief scientist for the World Health Organization (WHO).
But "that's still not sufficient because the national average positivity rate is about 15% -- in some cities like Delhi it's up to 30% or higher," she said Monday. "That means there are lots of people out there who are infected and not being detected just because of the capacity of testing ... we will know only later how many was really the number of people infected."
There are a few reasons for the insufficient testing, according to Bhramar Mukherjee, professor of biostatistics and epidemiology at the University of Michigan. The most obvious is that asymptomatic patients -- also called "silent infections" -- may simply never know they were infected, and so never get tested.
There are also different case reporting structures across different cities and states, and testing may be less accessible in rural areas. Poorer residents might not be able to afford the time off work to get tested, or to travel to a test center.
"All countries to some extent have faced this problem of accurately classifying Covid-related deaths, but I think in India the problem is quite acute," said Mukherjee.
But serology surveys, which test for antibodies in the immune system to indicate if someone has been exposed to the virus, give scientists a better measure of how many people may be infected in reality.
Previous national surveys showed the number of such people is "at least 20 to 30 times higher than what had been reported," said WHO's Swaminathan.
When applied to the latest reported figures as of Tuesday, that estimate could raise India's total to more than 529 million cases.
Even before the pandemic, India was undercounting its dead.
The country's underfunded public health infrastructure means that even in normal times, only 86% of deaths nationwide are registered in government systems. And only 22% of all registered fatalities are given an official cause of death, certified by a doctor, according to community medicine specialist Dr. Hemant Shewade.
The majority of people in India die at home or other places, not in a hospital, so doctors usually are not present to assign a cause of death -- a problem that has only deepened in the second wave, with hospitals out of space. With nowhere to go, Covid patients are now increasingly dying at home, in idling ambulances, in waiting rooms and outside overwhelmed clinics.
There are logistical problems too, like missing information in the national database or human error. And these issues are even more pronounced in rural areas.
The director of the National Centre for Disease Informatics and Research, a body within the government-run Indian Council of Medical Research (ICMR), said in a 2020 report in the Lancet journal that it was difficult to ensure individual states followed the guidelines to capture all confirmed and suspected Covid deaths.
"As per the existing law, NCDIR is not required to get data about suspected or probable deaths from states so I can't say whether deaths are being certified," he said.
As of Tuesday, India has reported nearly 198,000 coronavirus deaths. However, Mukherjee estimates Covid fatalities could be underreported by a factor of between two and five -- meaning the real death toll could be close to 990,000.
The number of mass funerals, cremations and bodies piling up have cast doubt on the official reported deaths in numerous cities these past few weeks. The discrepancies could partially be due to patients dying before they are tested, or having non-Covid factors listed as their cause of death, experts say.
"The real challenge with capturing Covid deaths is because the cause of death is often assigned a comorbidity like kidney disease or heart disease," said Mukherjee. "That's why many countries are now doing excess death calculations, excess mortality calculations, in the UK and United States."
These discrepancies are stark for those on the ground, as thousands die across the country every day.
"In Delhi, at least 3,000 people went to funerals in the last week," said Max Rodenbeck, South Asia Bureau Chief for The Economist, on Monday. "There is one crematorium in Delhi, which is a big land in the park, and (it is) building 100 new funeral pyres ... This, is again, in India's biggest city with the most attention. What happens beyond Delhi is pretty awful."
The underreporting could be partly why India was largely caught off guard by the second wave, said Mukherjee from the University of Michigan.
"If we had more accurate data in terms of cases, infections, as well as deaths, then of course, we'll be much more prepared and also anticipate the healthcare resource needs," she said. "(Faulty data) does not really change the truth. It only makes it worse for policymakers to anticipate the needs."
The second wave, which began in mid-March, hit the capital New Delhi hard, as well as the western state of Maharashtra. The union territory of Delhi, where New Delhi is located, was placed under lockdown on April 19. That lockdown has since been extended until May 3.
But cases are on the rise in other states, prompting some authorities to impose new restrictions in an attempt to avoid the kind of calamity seen in the capital.
The southern Karnataka state is imposing a 9 p.m. curfew for the next two weeks starting Tuesday, with only essential services allowed between 6 a.m. to 10 a.m., according to CNN affiliate News 18.
The northern state of Punjab also announced similar measures on Monday, including a night curfew and a weekend lockdown. "Urge you all to stay at home and step out only if absolutely necessary," tweeted the state's chief minister on Monday.
Meanwhile, states and local authorities are desperately waiting for aid to arrive from the central government and overseas. The US, UK, France, Germany, and Pakistan are among the countries that have stepped in to offer assistance and send much-needed medical equipment including ventilators and oxygen.
The first shipments of aid from the United Kingdom arrived in India on Tuesday, according to British Foreign Secretary Dominic Raab. "No one is safe until we are all safe," he tweeted, with pictures of the aid arriving.