The U.S. recently reached an important psychological milestone when the number of coronavirus vaccinations surpassing the number of COVID-19 cases, but the country is not out of the woods yet.

The rise of new variants or mutant strains of the coronavirus has proven to be a challenge in containing the spread and ensuring that vaccines are effective. The two most significant variants so far are out of the U.K. (the B117 strain) and South Africa (B.1.351), both of which are expected to become dominant in the U.S. in March.

“Essentially, we are in a race against time here,” Dr. Manish Garg, an emergency medicine physician and co-founder of the World Academic Council of Emergency Medicine, said on Yahoo Finance Live (video above). “These variants can be thought of probably as either tidal waves or as forest fires when they come. They’re basically borne from regions or areas where the rates are very, very high, and there are multiple countries around the world where they’re developing these variants and they’re coming in.”

‘Folks need to continue to wear masks’

There have been confirmed cases of both strains in the U.S. already, raising concerns about whether or not the current COVID-19 vaccines will be able to work against it.

Pfizer (PFE) and Moderna (MRNA), the companies behind the two vaccines currently being administered throughout the U.S., have said their shots are highly effective against the U.K. variant and less effective against the South African strain. Both companies are now working on developing booster shots directly targeting that variant.

Both variants are considered more transmissible than the original strain of the virus, meaning that until there is widespread vaccination, the best course of action is continuing practicing public safety measures.

“If we’re expecting that the B117 is the dominant strain that’s going to be in the United States in the next couple of months, then we need folks to get vaccinated,” Garg said. “Folks need to continue to wear masks publicly, make sure they’re in ventilated spaces, be outdoors, make sure that they’re physically distanced, do the hand washing — all of the things that we know are going to help.”

‘A lot of volume of patients who are still coming in’

It’s not clear if those who are fully vaccinated can still pass the virus on to others, which is why it is essential for everyone to keep masking until the country crosses the herd immunity threshold of roughly 75-80%.

Most importantly, Garg said, is to not let your guard down yet and ease back some of the restrictions, like reopening indoor dining, which many states have already begun doing.

“I can certainly tell you that I’m seeing a lot of volume of patients who are still coming in,” Garg said. “We have about a 10%-plus rate in the last week for all comers who were COVID positive. It’s a concern for us.”

According to Dr. Fauci, variants are able to form the longer the virus is rampant, meaning that containment and control is crucial, along with vaccinating people "as quickly as possible."

“This is the time when we really, really need to buckle down and bunker down and make sure that we’re observing really, really good protective measures, making sure that we can get the vaccine in,” Garg said.

But, he continued, “if we lax all of these and get more folks together — and these are people that have not had primary or secondary immunity either through the infection or through the vaccination — then we are going to probably recapitulate what we’ve already been seeing with further sickness, further illnesses, and more of these waves, and then potentially more variants being created.”

Adriana is a reporter and editor covering politics and health care policy for Yahoo Finance. Follow her on Twitter @adrianambells.