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Paris, Apr 17: Even as virologists zero in on the virus that causes COVID-19, a very basic question remains unanswered: do those who recover from the disease have immunity?

There is no clear answer to this question, experts say, even if many have assumed that contracting the potentially deadly disease confers immunity, at least for a while.

"Being immunised means that you have developed an immune response against a virus such that you can repulse it," explained Eric Vivier, a professor of immunology in the public hospital system in Marseilles.

"Our immune systems remember, which normally prevents you from being infected by the same virus later on."

For some viral diseases such a measles, overcoming the sickness confers immunity for life.

But for RNA-based viruses such as Sars-Cov-2 -- the scientific name for the bug that causes the COVID-19 disease -- it takes about three weeks to build up a sufficient quantity of antibodies, and even then they may provide protection for only a few months, Vivier told AFP.

At least that is the theory. In reality, the new coronavirus has thrown up one surprise after another, to the point where virologists and epidemiologists are sure of very little.

"We do not have the answers to that -- it's an unknown," Michael Ryan, executive director of the World Health Organization's Emergencies Programme said in a press conference this week when asked how long a recovered COVID-19 patient would have immunity.

"We would expect that to be a reasonable period of protection, but it is very difficult to say with a new virus -- we can only extrapolate from other coronaviruses, and even that data is quite limited."

For SARS, which killed about 800 people across the world in 2002 and 2003, recovered patients remained protected "for about three years, on average," Francois Balloux director of the Genetics Institute at University College London, said.

"One can certainly get reinfected, but after how much time? We'll only know retroactively."

A recent study from China that has not gone through peer review reported on rhesus monkeys that recovered from Sars-Cov-2 and did not get reinfected when exposed once again to the virus.

"But that doesn't really reveal anything," said Pasteur Institute researcher Frederic Tangy, noting that the experiment unfolded over only a month.

Indeed,several cases from South Korea -- one of the first countries hit by the new coronavirus -- found that patients who recovered from COVID-19 later tested positive for the virus.

But there are several ways to explain that outcome, scientists cautioned.

While it is not impossible that these individuals became infected a second time, there is little evidence this is what happened.

More likely, said Balloux, is that the virus never completely disappeared in the first place and remains -- dormant and asymptomatic -- as a "chronic infection", like herpes.

As tests for live virus and antibodies have not yet been perfected, it is also possible that these patients at some point tested "false negative" when in fact they had not rid themselves of the pathogen.

"That suggests that people remain infected for a long time -- several weeks," Balloux added. "That is not ideal."

Another pre-publication study that looked at 175 recovered patients in Shanghai showed different concentrations of protective antibodies 10 to 15 days after the onset of symptoms.

"But whether that antibody response actually means immunity is a separate question," commented Maria Van Kerhove, Technical Lead of the WHO Emergencies Programme.

"That's something we really need to better understand -- what does that antibody response look like in terms of immunity."

Indeed, a host of questions remain.

"We are at the stage of asking whether someone who has overcome COVID-19 is really that protected," said Jean-Francois Delfraissy, president of France's official science advisory board.

For Tangy, an even grimmer reality cannot be excluded.

"It is possible that the antibodies that someone develops against the virus could actually increase the risk of the disease becoming worse," he said, noting that the most serious symptoms come later, after the patient had formed antibodies.

For the moment, it is also unclear whose antibodies are more potent in beating back the disease: someone who nearly died, or someone with only light symptoms or even no symptoms at all. And does age make a difference?

Faced with all these uncertainties, some experts have doubts about the wisdom of persuing a "herd immunity" strategy such that the virus -- unable to find new victims -- peters out by itself when a majority of the population is immune.

"The only real solution for now is a vaccine," Archie Clements, a professor at Curtin University in Perth Australia, told AFP.

At the same time, laboratories are developing a slew of antibody tests to see what proportion of the population in different countries and regions have been contaminated.

Such an approach has been favoured in Britain and Finland, while in Germany some experts have floated the idea of an "immunity passport" that would allow people to go back to work.

"It's too premature at this point," said Saad Omer, a professor of infectious diseases at the Yale School of Medicine.

"We should be able to get clearer data very quickly -- in a couple of months -- when there will be reliable antibody tests with sensitivity and specificity."

One concern is "false positives" caused by the tests detecting antibodies unrelated to COVID-19.

The idea of immunity passports or certificates also raises ethical questions, researchers say.

"People who absolutely need to work -- to feed their families, for example -- could try to get infected," Balloux.

The Covid-19 pandemic has made an unprecedented impact on the Indian businesses, particularly small and medium enterprises (SMEs) and startups. According to a joint survey by FICCI and Indian Angel Network (IAN), the pandemic has hit the businesses of around 70% startups.

With uncertainty in the business environment and an unexpected shift in priorities of the government as well as corporates, many startups are struggling to survive, it says.

In a nationwide survey on the 'Impact of Covid-19 on Indian Startups' involving 250 startups, 70% participants said their businesses had been impacted by Covid-19 and around 12% had shut operations.

The survey shows only 22% startups have cash reserves to meet the fixed cost expenses over the next 3-6 months, and 68% are reducing operational and administrative expenses.

Around 30% of the companies said they would retrench employees if the lockdown was extended too long. The 43% startups have already started 20-40% salary cuts over April-June.

Over 33% startups said investors had put the investment decision on hold and 10% said the deals had been scrapped. Only 8% startups had received funds as per the deals signed before Covid-19 outbreak, the survey revealed.

The reduced funding has forced startups to put a hold on business development and manufacturing activities, which has resulted in loss of projected orders.

The survey highlights the need of an urgent relief package for startups, including possible purchase orders from the government, tax relief and swifter tax refunds, and immediate fiscal support measures, including grants, soft loans and payroll grants.

Besides 250 startups, 61 incubators and investors also participated in the survey.

While 96% of investors accepted that their investments in startups had been impacted by Covid-19, 92% said their investments in startups would continue to be low over the next six months.

Around 59% investors said they would prefer to work with the existing portfolio firms in the coming months. Only 41% said they would consider new deals.

"A comparison of priority investment sectors before and during Covid-19 shows 35% investors are now looking at investments in healthcare startups, followed by EdTech, AI/Deep Tech, FinTech and Agri," said the survey.

Around 44% incubators surveyed said their day-to-day operations had been considerably hit by Covid-19. Most incubators are now supporting their portfolio firms by providing them virtual platforms to interact with mentors, investors and industries.

Dilip Chenoy, FICCI Secretary General, said, "The startup sector is stressed for survival at the moment. The investment sentiment is also subdued and is expected to remain so in the coming months. Lack of working capital and cash flows may lead to major layoffs over the next 3-6 months."

Indian startups needed an enabling ecosystem and flow of funds to continue operations, the survey said.

Padmaja Ruparel, President, Indian Angel Network & Co-Chair of FICCI Startup Committee, said, "In these uncertain times, as investors, we must play an important role to provide the Indian startups funding, mentoring and hand-holding support to stay afloat and come out at the other end of this crisis."

To that end, IAN recently announced a debt fund to help IAN portfolio companies raise working capital and ensure business continuity by partnering with debt providers.

This must be replicated on a wider scale, so a larger number of startups are provided the capital support to make it during these tough times, Ruparel said.

In a study conducted in 117 countries, researchers have found that the world is experiencing the most dramatic reduction in the seismic noise (the hum of vibrations in the planet's crust) in recorded history due to global COVID-19 lockdowns.

Measured by instruments called seismometers, seismic noise is caused by vibrations within the Earth, which travel like waves and the waves can be triggered by earthquakes, volcanoes, and bombs - but also by daily human activity like travel and industry.

This quiet period was likely caused by the total global effect of social distancing measures, closure of services and industry, and drops in tourism and travel, the study published in the journal Science, reported.

The new research, led by the Royal Observatory of Belgium and five other institutions around the world including Imperial College London (ICL), showed that the dampening of 'seismic noise' caused by humans was more pronounced in more densely populated areas.

"Our study uniquely highlights just how much human activities impact the solid Earth, and could let us see more clearly than ever what differentiates human and natural noise," said study co-author Stephen Hicks from ICL in the UK.

For the findings, the research team looked at seismic data from a global network of 268 seismic stations in 117 countries and found significant noise reductions compared to before any lockdown at 185 of those stations.

Researchers tracked the 'wave' of quietening between March and May as worldwide lockdown measures took hold.

The largest drops in vibrations were seen in the most densely populated areas, like Singapore and New York City, but drops were also seen in remote areas like Germany's the Black Forest and Rundu in Namibia.

Citizen-owned seismometers, which tend to measure more localised noise, noted large drops around universities and schools around Cornwall, UK and Boston, US - a drop in noise 20 per cent larger than seen during school holidays.

The findings showed that countries like Barbados, where lockdown coincided with the tourist season, saw a 50 per cent decrease in noise.

"The changes have also given us the opportunity to listen in to the Earth's natural vibrations without the distortions of human input," the study authors wrote.

Earlier in April, a study published in the journal Nature, reported at least a 30 per cent reduction in that amount of ambient human noise since lockdown began in Belgium.