After weeks of stay-at-home orders and closed businesses, Americans are looking for answers as to how their life can return to some semblance of normalcy with the coronavirus still infiltrated within communities.

Health experts turned their sights on one form of testing in recent weeks to guide state and local officials during reopening.

Antibody testing strays from the initial Covid-19 test because instead of determining who has the infection, it determines who came in contact with the novel virus and if they developed some immunity against it.

“It will basically tell us who in the population has been exposed to the virus, whether or not they had symptoms. They could be asymptomatic, they could be mildly symptomatic,” Dr Danielle Ompad, an epidemiologist and professor at New York University School of Global Public Health, told The Independent.

Already two studies were rolled out in California with residents in Santa Clara and Los Angeles. Roughly about 1.9 per cent of the population was estimated to have been exposed to Covid-19 in Santa Clara. In Los Angeles, the researches estimated 2.5 per cent to just over 4 per cent had antibodies for the virus.

The data could suggest the coronavirus was not as deadly as initially anticipated given the percentage of people exposed compared to the hospitalisation and death rate. But it could also mean “you have potentially a lot of people who are vulnerable,” Dr Ompad explained.

Scientist clashed over the results, though, warning the population false positives could impact the results, Buzzfeed News reported.

How antibody testing works

Antibodies are the body’s defence mechanism against a virus.

“The first round of defence that you get from immune cells is called IgM,” Dr Ompad said. “It’s not specific, it’s more general.”

IgM will appear within the first couple days of infection as a “general” defence but will later disappear. Then IgG antibodies develop as the body works to clear the infection and will remain in the body as long-term protection in case the virus makes a reappearance.

“IgG fits like a glove and it’s very specific to whatever that foreign invader is,” Dr Ompad added.

Testing positive for just IgG means the person has been exposed to the virus. If they test positive for both IgG and IgM that means they’ve been exposed to the virus and it was in recent days.

Unlike the first coronavirus tests, which entailed swabbing inside the nose for the virus, the antibody tests typically involve taking the blood and looking specifically for IgG and IgM related to Covid-19.

Using antibody tests to estimate how many people have been exposed to the novel virus could be an important step for health and state officials when determining how to open up specific areas of the country.

“If we don’t have an understanding of how many people in the community have been exposed, it makes it very difficult to make decisions about how to move forward with any non-pharmaceutical interventions we’ve been using: social distancing, wearing masks, everybody staying home,” Dr Ompad said. ”The more information we have, the better we can predict the impact of changing any of those interventions.”

If a low number of residents were exposed to the virus based on antibody testing while hospitalisation and death rates were on the incline, that would indicate an area should adhere to social distancing and stay-at-home measures. But a declining hospitalisation and death rate combined with information on the number of residents exposed could push areas towards reopening.

These decisions hindered on a balancing act of the data in each community.

New York state’s “aggressive” antibody testing

Following on the heels of parts of California was New York state announcing “the most aggressive” statewide antibody testing initiative.

The state was hit the hardest in the US with the coronavirus after infection, hospitalisation, and death rates soared. But New York has since reached its apex for the virus and has started talks about how to reopen.

New York Governor Andrew Cuomo said Wednesday the state was not yet ready to reopen, but it was working to determine how the novel virus has infiltrated the communities through antibody testing and contact tracing.

To start the initiative, the state would be testing 3,000 residents at random this week. It was not revealed how the health officials would select the random individuals.

“Any plan to start to reopen the economy has to be based on data and testing, and we have to make sure our antibody and diagnostic testing is up to the scale we need so we can safely get people back to work,” Mr Cuomo said in a statement.

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Mr Cuomo added that Germany, which has a population of 83m compared to New York’s 19.5m, was using the same sample size of residents for its own antibody testing initiative.

“Large-scale antibody testing will help determine the percentage of the population that is now immune to the virus,” the governor added. But it’s worth noting researchers have yet to determine if the body could form an immunity against Covid-19 like other viruses.

Maria Van Kerkhove, the World Health Organisation‘s (WHO) technical lead on COVID-19, reiterated on Monday evidence has yet to show people could develop immunity from the virus.

“Right now, we have no evidence that the use of a serological test can show that an individual has immunity or is protected from reinfection,” she said. Serological tests are ones that test antibodies in the blood.

Does a positive antibody result mean you’re ‘safe’

Short answer is “no”.

Health experts have yet to confirm the virus would cause immunity for an individual or “herd immunity”, where a large percentage of people in a community or state have developed antibodies for the novel virus.

In South Korea, which halted the novel virus early into the infection, it reported at least 91 residents who recovered from the virus tested positive again.

Despite potential for someone becoming reinfected, countries have discussed issuing “immunity passports” to residents after they tested positive for antibodies, but this comes, again, without evidence someone can develop full immunity from Covid-19.

Chile became the first country to announce it would issue these so-called “passports” to residents in a bid to reopen its country.

Paula Daza, an undersecretary in the Chilean Health Ministry, said more than 4,600 people recovered from the coronavirus and would be eligible for these passports, which would be physical or digital cards, The Washington Post reported. Anyone who holds a passport would be exempt from restrictions and could start work again.

Other countries including the UK, Germany, and Italy have considered the passport but have yet to enforce one for residents.

The WHO has distanced itself from specifically advising for or against immunity passports, but the organisation did warn no more than 2 to 3 per cent of the global population has likely developed antibodies against the virus.

Scientists are now working to determine what kind of protection is given with Covid-19 antibodies and for how long. Until then, a balancing act between following the data and testing residents would be necessary for officials to safely open up their areas, instead of solely relying on antibody results.