New Orleans EMS Shakera Brown gives Deidra Howard her first shot of a COVID-19 vaccine in New Orleans, March 30.

Just more than 380,000 people in Louisiania have tested positive for the novel coronavirus since COVID-19 was first detected in the state a year ago, the latest statistics say. And, if you count those who were infected but not tested, some 760,000 to 1.5 million could have gotten the virus -- as much as one third of the state, some estimates suggest.

But state health officials and medical experts say those people should still be vaccinated. 

Why should people who have been through the virus and had a natural immune response need vaccination? The short answer, based on scientists' evolving understanding of the virus, is that natural immunity probably isn't good enough, because it doesn't appear to last very long. And reinfection is a risk, though an infrequent one currently.

"From what we know now, the vaccine confers greater protection than natural infection does," said Dr. Joseph Kanter, state health officer and top medical official in the state Department of Health. 

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In clinical trials, the Pfizer and Moderna vaccines were shown to have 94% to 95% effectiveness at preventing illness and prevented 100% of deaths from COVID-19.

The newer, single-shot Johnson and Johnson vaccine had lower effectiveness against preventing illness -- 66% -- but also prevented all deaths from COVID-19 in its clinical trial. Scientists think the vaccine is significantly easing the symptoms in those who are sickened by the virus.

While scientists are still trying to determine if people who have gotten the vaccine will need a booster shot at some point, the U.S. Centers for Disease Control and Prevention has roughly estimated that natural immunity may last around three months after infection.

Kanter and others said they have seen or had reported cases where people have been re-infected once and even twice after the initial infection. 

"We've seen people become re-infected with symptomatic infections after they have been proven to infected in the past, and we don't have any data to suggest that after 90 days you still have protective antibodies to the virus," said Dr. Ralph Dauterive Jr., vice president of medical affairs for Ochsner Medical Center in Baton Rouge.  

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Herd immunity means that a large of enough share of the population has immunity so a virus has no place to go and forces the spread of the virus to essentially halt though not necessarily that the virus has been eradicated. 

State and federal health officials say that herd immunity should be reached when immunity has been conferred to 70% to 80% of the population. In Louisiana, nearly 16% of the population had been fully vaccinated as of midday Monday, state statistics say. 

Like Kanter and Dauterive, the CDC says reinfections have been infrequent. But the federal health agency adds that the probability of reinfection is expected to increase with time because of "waning immunity and the possibility of exposure to virus variants."

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The agency notes that cases of reinfection with variants have been reported in South Africa, Brazil and the United Kingdom. 

"Continued widespread transmission makes it more likely that reinfections will occur," the CDC says.

That reality, the CDC says, provides all the more reason to keep wearing a mask and employing other social distancing requirements. 

Some recent studies, however, would appear to challenge the view the natural immunity wanes after three months, which earlier research had supported.

Among the listed sources for the CDC's current recommendations are a handful of studies that suggest natural immunity may last for five to eight months.

One recent study of more than 43,000 people in Qatar who had previously had the virus found just 0.1% had been re-infected up to seven months after their initial infection.

The results, the authors say in a January preprint report that had not been peer-reviewed, suggested the natural immunity during that time could confer greater than a 90% protection against reinfection for at least seven months.

Published in March, a separate South Korean study of 58 largely young adults found high antibody positivity up to eight months after asymptomatic or mildly symptomatic infections.

Susan Hassig, a Tulane epidemiologist, said the devil is often in the details of these kinds to studies, however, which can undercut some of their conclusions. 

The Qatar study, for instance, may not have captured all reinfections because it relied on people who were diagnosed with a reinfection "incidentally through random or routine testing, or through contact tracing." They weren't all being tested regularly to capture potentially other infections. 

The South Korean study relied on a small sample, a fact the authors themselves acknowledged.

Hassig said scientists are still in the early phases of gathering data on the virus and understanding how it works. Each study is a piece of a broader picture that is still developing.

"Ultimately, we're going to have a picture, but it's a pretty big jigsaw puzzle that we're working on right now," she said.  

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