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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 GST Council is unlikely to make major changes in the indirect tax structure at its next meeting slated mid June.

A top government source said that the Centre is not in favour of increasing tax rates on any goods or service as it could further impact consumption and demand that is already suppressed due the COVID-19 pandemic and lockdown.

It was widely expected that the GST Council could consider raising tax rates and cess on certain non-essential items to boost revenue for states and the Centre. Several states have reportedly taken an over 80-90 per cent hit in GST collections in April, the official data for which has not yet been released by the Centre.

"The need of the hour is to boost consumption and improve demand. By categorising items into essential and non-essential and then raising taxes on non-essential is not what Centre favours. But, the issue on rates and relief will be decided by the GST Council that is meeting next month," the finance ministry official source quoted above said.

The GST Council is chaired by the Union finance minister and thus the views of the Centre play out strongly in the council meetings.

However, the Council will also have to balance the expectations of the states whose revenues have nosedived after the coronavirus outbreak and wide scale disruption to businesses while they have still not been paid GST compensation since the December-January period.

To the question of wider scale job losses in the period of lockdown as businesses get widely impacted, the official said that the Finance Ministry has asked the labour ministry to collect data on job losses during Covid-19 and is constantly engaging with the ministry to oversee job losses and salary cuts.

On restrictions put on Chinese investment in India, the official clarified that no decision had yet been taken to restrict China through the Foreign Portfolio Investment (FPI) route.

Asked about monetising government debt, the official said that the issue would be looked at when we reach a stage. It has not come to that stage yet.

In the government's over Rs 20 lakh crore economic package, the official defended its structure while suggesting that comparisons with the economic packages of other countries should not be drawn as India's needs were different from others.

"We have gone in more reforms that is needed to give strength to the economy. This is required more in our country," the official source said.

With the scrapping of Mitron and Remove China Apps from its Play Store gaining a lot of attention in India, Google on Thursday said that it removed a video app "for a number of technical policy violations", while adding that it also does not allow an app that "encourages or incentivizes users into removing or disabling third-party apps".

Both the apps became immensely popular in India within a short span of time due to the prevailing anti-China sentiment amid border tensions between India and China in Ladakh and calls by Indian activists to boycott Chinese products.

Reports suggested that the Mitron app is a repackaged version of TicTic, which is a TikTok clone.

The Remove China Apps was designed to help users identify applications of Chinese origin.

Without naming the apps, Google hinted that the Mitron app may make a comeback on the Play Store once it fixes some technical issues, but the chances of the Remove China Apps are thin.

"We have an established process of working with developers to help them fix issues and resubmit their apps. We've given this developer (of the video app) some guidance and once they've addressed the issue the app can go back up on Play," Sameer Samat, Vice President, Android and Google Play, said in a statement.

Google said that its Android app store was designed to provide a safe and secure experience for the consumers while also giving developers the platform and tools they need to build sustainable businesses.

Samat said that Google Play recently suspended a number of apps for violating the policy that it does not allow an app that "encourages or incentivizes users into removing or disabling third-party apps or modifying device settings or features unless it is part of a verifiable security service".

"This is a longstanding rule designed to ensure a healthy, competitive environment where developers can succeed based upon design and innovation. When apps are allowed to specifically target other apps, it can lead to behaviour that we believe is not in the best interest of our community of developers and consumers," Samat said.

"We've enforced this policy against other apps in many countries consistently in the past - just as we did here," he added.