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The more transmissible Delta variant has spread to almost every state in the US, fueling health experts’ concerns about Covid-19 spikes.

The variant is expected to become the dominant coronavirus strain in the US, the director of the Centers for Disease Control and Prevention said. And with half the US still not fully vaccinated, doctors say it could cause a resurgence of Covid-19 in the fall – just as children too young to get vaccinated go back to school.

In Los Angeles County, the pace of Delta’s spread has prompted officials to reinstate mask guidance for public indoor spaces – regardless of vaccination status.

The new, voluntary mask guidance is needed until health officials can “better understand how and to who the Delta variant is spreading,” the county’s department of public health said.

The Pfizer/BioNTech vaccine has shown to be 88% effective against symptomatic infections caused by the Delta variant – two weeks after the second dose. Those who received only one dose have significantly less protection.

And Moderna’s vaccine was found in lab experiments to work against new variants such as the Delta strain, the company said Tuesday. Researchers used serum samples from eight participants taken a week after they received their second dose of vaccine.

But the spread of coronavirus is outpacing vaccinations, the World Health Organization said. And the longer coronavirus spreads among unvaccinated people, the more opportunities it has to mutate into more troubling variants.

Now the Gamma variant has been shown to be more resistant to vaccines and antibody treatments. Last week, the WHO said even those who are fully vaccinated should wear masks in places with high rates of Covid-19 spread.

Many states have not reinstated mask mandates for the upcoming school year, including New Jersey – where masks will not be required “unless the district decides to make it protocol,” Gov. Phil Murphy said Monday.

But that could change if the situation gets worse, he said.

How to help kids go back to school in-person, safely

For parents of children ages 12 and up, the time to vaccinate them for school is now, said Dr. Ashish Jha, dean of Brown University’s School of Public Health.

“My message is really clear: You should get your kids vaccinated,” he told CNN on Monday.

“I have two teenagers, both of whom have gotten vaccinated. The safety profile on these vaccines are really quite extraordinary, and they’re much, much safer than getting Covid.”

There have been extremely rare reports of myocarditis, or inflammation of the heart muscle, and pericarditis, or inflammation of the outer lining of the heart, after the Pfizer/BioNTech or Moderna vaccines, the CDC says.

Symptoms can include shortness of breath, chest pain or the feeling of a fast heartbeat. “Patients can usually return to their normal daily activities after their symptoms improve,” the agency says.

“The known and potential benefits of COVID-19 vaccination outweigh the known and potential risks, including the possible risk of myocarditis or pericarditis,” the CDC says.

“CDC continues to recommend COVID-19 vaccination for everyone 12 years of age and older.”

The Delta strain has been associated with 2.61 times the risk of hospitalizations compared to the previously dominant strain in the UK.

In the US, hot spots such as Missouri are seeing increased hospitalizations due to the Delta variant. Those hospitalized now are younger than those hospitalized during the winter Covid-19 surge.

“People in their late teens and even early 20s are being hospitalized and needing the use of ventilators,” said Katie Towns, acting director of the Springfield-Greene County Health Department.

The risk-benefit analysis between getting Covid-19 vs. getting vaccinated is a “no-brainer,” Jha said.

“Getting Covid itself is so much worse,” he said.

With some schools starting in early August, parents of children ages 12 and older would need to get their adolescents vaccinated soon to be protected before the school year.

The Pfizer/BioNTech vaccine is the only one authorized for children 12 and up. It requires two doses, spaced three weeks apart, and the vaccine doesn’t fully kick in until two weeks after the second dose.

So it’ll take five weeks from the first dose of the Pfizer/BioNTech vaccine to be fully vaccinated.

Some vaccines offer years of protection, study shows

A new study suggests the two-dose Pfizer/BioNTech vaccine should keep an immune response up for years – unlike vaccines for the flu that need a yearly booster.

The human body produces antibodies to attack and neutralize an invader such as a virus, but antibodies typically die off over time. To ensure a long-term response, the body needs to be able to make more antibodies that can specifically respond to certain viruses via B-cells.

Researchers at Washington University in St. Louis found people who got both doses of the Pfizer/BioNTech vaccine had little factories called germinal centers that make B-cells that should specifically recognize the novel coronavirus, meaning there’s a possibility for long-lasting protection, according to a study published in the journal Nature.

Covid-19 – not the vaccine – can be dangerous for pregnant women

With many popular myths about Covid-19 vaccines, health experts want to reiterate that the vaccines will not affect fertility.

And pregnant woman are at much greater risk of complications from Covid-19 than the Covid-19 vaccines, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

“Tens and tens and tens of thousands of people” have received the vaccine while pregnant and before getting pregnant, Fauci said Monday.

And unlike the vaccines, Covid-19 can be especially dangerous during pregnancy, Fauci said.

“The mother can have an adverse pregnancy event, as can the fetus” with Covid-19, he said.

“The best thing one can do to protect yourself is to actually get vaccinated.”

CNN’s Jacqueline Howard, Alexandra Meeks, Christina Bowllan, Naomi Thomas, Maggie Fox, Elizabeth Cohen, Lauren Mascarenhas and Stella Chan contributed to this report.