Just as Covid-19 numbers were dropping, a concerning strain detected in South Africa is now in the US

Now, one of the most worrisome strains -- first found in South Africa -- has been detected for the first time in the US. On Thursday, South Carolina officials confirmed two cases of the more contagious strain, also known as B.1.351.

Those two people, both adults, have no known travel history or connection to one another, state health officials said.

This strain has concerned scientists because it seems to more transmissible than other versions of the virus, and because it might evade some of the immune protection offered by antibodies.

Experts say they believe vaccines will still be effective against the variant, and the Centers for Disease Control and Prevention said "we have no evidence that infections by this variant cause more severe disease."

But for anyone getting lax about safety precautions, the confirmation of B.1.351 in the US should be a big wake-up call.

"The arrival of the SARS-CoV-2 variant in our state is an important reminder to all South Carolinians that the fight against this deadly virus is far from over," said Dr. Brannon Traxler, interim public health director for the state's health department.

The variant has been detected in more than 30 other countries, according to the World Health Organization.

Why we're at a critical time

New variant strains spreading in the US could make even everyday activities more dangerous.

A highly contagious variant first spotted in the UK have been detected in at least 28 states, according to the CDC.

And Minnesota officials recently announced the first US case of another variant that was first detected in Brazil. That person had recently traveled from Brazil.

"The variants that have been identified recently seem to spread more easily," CDC Director Dr. Rochelle Walensky said.

"They're more transmissible, which can lead to increased number of cases and increased stress on our already overtaxed system."

With several days still left to go, January has already been the deadliest month of the entire pandemic in the US, with more than 80,000 deaths.

In less than one year, Covid-19 has claimed more than 430,000 US lives. And a composite forecast by the CDC projects another 84,000 deaths by February 20.

Why some vaccines are 'sitting on the shelves'

The US is still grappling with vaccine allocation and distribution problems. Only about half the vaccine supply that has been distributed in the US has been administered, federal data shows, and it could be months until every American who wants a vaccine can get it.

During a CNN town hall Wednesday, Walensky explained some of the reasons why.

Some of the 47 million doses distributed just arrived in states, while others are still going through a dayslong distribution process to make it to their final destination, she said.

"So there is some delay from the time that they're distributed and to the time that they could possibly be administered," Walensky said. There's also a delay in reporting the administrations, she said.

"And then there are some doses that we need to make sure, for the four- or five-day window that we give people in order to get their second shot ... we need to make sure that that's available for them when they return for their second shot," she added.

But even with those considerations, there are still "millions" of doses left "sitting on the shelves," she said. "That's one of the bottlenecks and one of the ways that we have to get resources to the states, to make sure that they can quickly administer the vaccines that are on their shelves," she said.

Service members and retirees might help with vaccinations

The Federal Emergency Management Agency plans to request as many as 10,000 service members to support vaccine administration.

Service members could be deployed at up to 100 sites nationwide to help boost administrations, a FEMA spokesperson said.

The federal government is also amending rules to help broaden the list of people who can administer vaccines, White House Covid-19 response coordinator Jeff Zients said Wednesday.

The Department of Health and Human Services "will amend the current Public Readiness and Emergency Preparedness Act -- otherwise known as the PREP Act -- to permit doctors and nurses who have recently retired or become inactive to administer shots and to permit anyone currently licensed to vaccinate in their state to administer shots across state lines," Zients said.

You still need to wear a mask after getting vaccinated

Once Americans do get both doses of their Covid-19 vaccine, they absolutely should not ditch safety precautions, Dr. Anthony Fauci said.

First, it takes a while for vaccines to kick in. Maximum immunity starts around 10 days to "two weeks and beyond" the second dose, said Fauci, the director of the National Institute of Allergy and Infectious Diseases.

Even after that, there's the potential to spread the virus to others.

"People ask, 'Why should I even have to wear a mask after I get my second shot? And the reason is very clear," Fauci said.

After vaccination, "you can conceivably get infected, get no symptoms and still have virus in your nose, which means you have to wear a mask to prevent you from infecting someone else."

On the flip side, Fauci said, "you may not be totally protected yourself."

Health experts say Americans should keep wearing masks through at least through spring, since we're still months away from herd immunity through vaccination.

Think again about any travel plans

Early vaccinations should not be viewed as "a free pass to travel," Fauci said.

"It is not a good idea to travel. Period," Fauci told CNN's town hall.

"If you absolutely have to travel and it's essential, then obviously one would have to do that. But we don't want people to think because they got vaccinated, then other public health recommendations just don't apply."

No signs of major safety concerns with vaccines

As for the vaccines themselves, here's what scientists know so far.

Severe allergic reactions, known as anaphylaxis, are not a problem, Walensky said.

"Based on our most recent data ... we found that there were 2.1 cases of anaphylaxis per million administered doses of Moderna, and 6.2 cases of anaphylaxis per million doses administered of Pfizer," she said during a White House Covid-19 response team briefing.

"Let me be clear: These are rare treatable outcomes, and the Covid-19 vaccines are safe," she said.

It's also important to put the numbers into context and consider that the risk of getting sick with the virus is "much higher" than the risk of allergy from the vaccine, the CDC director added.

Other mild side effects, like pain in the arm, fatigue and muscle aches after the shot are "all normal and expected," Walensky said.

In fact, they're usually a good sign, the CDC director said.

"These symptoms mean that your immune system is revving up, and the vaccine is actually working."