(CNN) Two coronavirus strains first detected in California are now officially "variants of concern," according to the US Centers for Disease Control and Prevention.
The variants may be about 20% more transmissible, the CDC said, citing early research. Some Covid-19 treatments may also be less effective against the strains. Still, the CDC didn't say that vaccines would stop working against them.
In laboratory studies, antibodies from vaccinated people do seem to be less effective at neutralizing the strains. But lower levels of antibodies may still be enough to protect against Covid-19, especially severe cases. Certain immune cells can also help protect against disease -- not just antibodies.
No coronavirus variants currently rise to the US government's highest threat level, "variant of high consequence." Coronavirus strains shown to significantly reduce vaccine effectiveness would fall under that category.
Still, health officials are concerned that some treatments may not work as well against the variants, which are officially called B.1.427 and B.1.429. Scientists have been monitoring the strains closely in California over the past few months.
The US Department of Health and Human Services recently halted shipments of a Covid-19 antibody treatment to California, Nevada and Arizona, where the variants are circulating widely.
The agency cited concerns that the treatment, which is made by Eli Lilly and Company and called bamlanivimab, may be less effective against the strains.
Officials said that another Covid-19 therapy made by Eli Lilly, a combination of bamlanivimab and another drug, etesevimab, can still be ordered. Early results show the drug combination may significantly reduce the risk of hospitalization and death. A different antibody treatment made by Regeneron Pharmaceuticals was also not affected by the HHS action.
Both of those therapies are "cocktails" that combine two different lab-made antibodies. That's thought to make them more resilient to emerging strains of the coronavirus. If a mutation allows the virus to evade one antibody, it may still be susceptible to the other.
Eli Lilly said Tuesday that it is continually monitoring the Covid-19 environment for variants and testing its therapies against a "wide array of emerging mutations and variants."
"It has always been our view that additional antibodies from Lilly and others will need to be developed to address the evolution of the virus, including emerging variants that can differ by country or even by state," Eli Lilly said in a statement emailed to CNN on Tuesday.
"In fact, this is what drove our work on bamlanivimab and etesevimab together and continues to underpin our strategy moving forward."