Here is a FAQ answering that and other questions The Day has been hearing recently.

How much natural immunity against COVID-19 do people get from a prior infection, and why can't natural immunity be used to opt out of vaccine and testing mandates?

Doctors and researchers generally agree that people who previously were infected with COVID-19 and also got vaccinated are the most protected. But the science is still evolving on how much immunity prior infection confers — which may depend on the severity of the infection and the coronavirus variant circulating — and how long protection lasts.

"It's been very difficult to ascertain how the vaccine and natural immunity compare to each other," Dr. Shira Doron of Tufts Medical Center told NBC10 Boston last month. Centers for Disease Control and Prevention spokesperson Kristen Nordlund recently told Kaiser Health News, "We hope to have some additional information on the protectiveness of vaccine immunity compared to natural immunity in the coming weeks."

In a study released Aug. 6, the CDC looked at Kentucky adults who were infected with the coronavirus in 2020, and people who were and weren't reinfected in May or June of this year. The study found that unvaccinated people were 2.34 times more likely to be reinfected.

One of the limitations the authors listed is that "persons who have been vaccinated are possibly less likely to get tested. Therefore, the association of reinfection and lack of vaccination might be overestimated."

That is the opposite of a limitation that Emory University biostatistician Natalie Dean shared about a different study: Previously infected people may be less likely to get tested than vaccinated people because they think they're immune.

She talked to the journal Science about a study Israeli researchers released in August based on the database of a health care organization that enrolls about 2.5 million Israelis.

Comparing more than 32,000 people who either got their second Pfizer dose or were infected in January or February, one model found the risk of developing symptomatic COVID-19 was 27 times higher among the vaccinated than those with a previous infection, and the risk of hospitalization eight times higher. But the latter was just based on eight hospitalizations among the fully vaccinated and one of a previously infected person.

Using a model that included people who were infected in 2020, the researchers found evidence of waning natural immunity, but the risk of symptomatic disease was still seven times higher for vaccinated but never-infected people.

This study hasn't yet been peer-reviewed, and the preprint server medRxiv cautions that the medical research "has yet to be evaluated and so should not be used to guide clinical practice."

Dr. Richard Martinello, medical director for infection prevention for Yale New Haven Health, said more studies will need to be done to confirm the findings. He said we've always known people develop immunity after infection, but the question is whether it's good enough.

Researchers from the Yale School of Public Health analyzed data from close relatives of the COVID-19 virus to model the risk of reinfection over time. They said reinfection "under endemic conditions would likely occur between 3 and 63 months after peak antibody response, with a median of 16 months."

"Those who have been naturally infected should get vaccinated. Previous infection alone can offer very little long-term protection against subsequent infections," lead author Jeffrey Townsend said. This study was published in The Lancet Microbe on Oct. 1.

A Cleveland Clinic study released in June, which hasn't been peer-reviewed, found that 99.3% of employee infections between Dec. 16 and May 15 occurred among those who were unvaccinated and not previously infected. In those five months, COVID-19 didn't occur among any of the 2,579 previously infected individuals, including 1,359 people who were unvaccinated.

Some organizations in Connecticut responded to questions about why they opted not to allow prior infection as an exemption from vaccine and testing mandates.

"The science on immunity from natural infection with COVID is evolving. Some people develop very strong immunity and others don't," Department of Public Health Commissioner Dr. Manisha Juthani said in an email. "Vaccination after getting COVID reliably produces immunity which is why we continue to recommend getting vaccinated despite having had COVID. This recommendation may change over time as we evaluate more scientific data."

Martinello said Yale New Haven Health didn't consider natural immunity as an exemption, because natural immunity is not as consistently established as immunity conferred by a vaccine.

A University of California Irvine School of Medicine professor challenged the UC system's vaccine mandate, saying he had immunity from a prior infection, but a federal judge upheld the vaccine requirement in a ruling on Sept. 30.

The Michigan hospital system Spectrum Health said last month it would grant an exemption from its employee vaccine mandate to people who have a positive PCR or antigen test for COVID-19 plus a positive antibody test from the past three months, The Detroit News reported.

Some other countries accept a certificate of recovery in addition to proof of vaccination or a negative test: For both the EU Digital COVID Certificate and the Green Pass in Israel, a certificate of recovery is valid for six months.

How do school nurses handle students with COVID-19 symptoms?

School nurses here are relying on guidance from DPH, the state Department of Education and Ledge Light Health District.

Addendum 5 from DPH and the education department, which has been revised multiple times, says an individual who has COVID-19 symptoms but hasn't had close contact with a coronavirus-positive person should stay home, notify the school immediately and get tested.

If at school, "Students should remain masked, adhere to strict physical distancing, be assessed by the school nurse or school medical advisor (if available), stay in the isolation room (with adult supervision), until picked up to go home. Parents should be instructed to consult a healthcare provider and have the student tested."

Students who test negative can return to school after 24 hours with no fever, or can return without test results if a health care provider "provides documentation with a specific, confirmed alternate diagnosis."

The addendum has different protocols for close contacts, which depend on vaccination status and whether symptoms are present.

Annie Ryan, nurse at Nathan Hale Arts Magnet School in New London, said that about two weeks ago, she had to send home 17 kids in two days.

"I explain it to the parents," she said, "and some of them understand, and some of them are screaming in your face, like, 'Why did you send my kid home, because they just have a cold?'"

Ryan is president of the union that represents nurses for the Visiting Nurse Association of Southeastern Connecticut, which supplies nurses for schools in New London, Waterford, East Lyme and Groton.

Maria Kostopoulos, nurse at Clark Lane Middle School in Waterford, said one primary symptom or two secondary symptoms is enough for her to second a kid home. She said primary symptoms are fever, chills, loss of taste or smell, uncontrolled cough and shortness of breath.

Secondary symptoms include things like a sore throat, congestion, runny nose, nausea, headache and fatigue.

"The symptoms are very vague, so it's hard to figure out who to send home and who not to send home," Kostopoulos said, "so a lot of it goes with if you know these children's medical history. Are they prone to allergies?"

"It's multifaceted, and it's dealing with families when they're fragile and scared, and you don't know what their situations are at home," she said. Sending kids home is a hardship on parents, especially if they have to wait a few days to get a test scheduled and then a few days to get results.

Kostopoulos said what's striking is boards of education "can adopt the policies in their own way, with a little bit of discretion," so protocols vary town to town.

"I don't think there's a one size fits all (approach)," Lyme-Old Lyme Superintendent Ian Neviaser said. He added, "It really depends on the specific situation. We don't require, per se, the student provide a negative COVID test in every situation, because that would be in some cases a little ridiculous. If a kid has one symptom and it goes away immediately, the chances of them having COVID are pretty slim."

Where can I get tested?

Ledge Light Health District is offering free COVID-19 testing four days a week at four locations, for symptomatic and asymptomatic people, with no appointment required.

Testing is Tuesdays from 3 to 7 p.m. at the City of Groton Municipal Building, Wednesdays from 8 to 11 a.m. at the Waterford Public Library, Thursdays from 4 to 7 p.m. at Stonington Human Services and Fridays from noon to 5 p.m. at the New London Senior Center.

Uncas Health District and UCFS Healthcare are offering free drive-thru testing at Dodd Stadium in Norwich on Mondays, Tuesdays, Thursdays and Fridays from 3 to 7 p.m.

Through Yale New Haven Health, people can get tested at Lawrence + Memorial Hospital in New London or Mohegan Sun. To schedule an appointment, visit covidtesting2.ynhhs.org or call (833) 275-9644. Martinello said when people get tested because they're sick, the results come back within 24 hours more than 99% of the time.

CVS test scheduling is available at bit.ly/cvscvdtst, but not every site offers rapid testing or testing for asymptomatic people, and appointments may not be available the same day or the next day.

What do I do if I lose my COVID-19 vaccine card?

There are a few options to obtain proof of vaccination, such as contacting your vaccination provider to request a copy. If you got vaccinated through Yale New Haven Health or Hartford HealthCare, you can respectively log on to MyChart or MyChartPLUS to download your vaccination record.

You can log on to the Vaccine Administration Management System, or VAMS, if you scheduled your appointment that way.

Immunization records also are available from the CT Immunization Information System, or CT WiZ, by visiting bit.ly/ctwizportal or calling (860) 509-7929.

e.moser@theday.com