Despite a dip in U.S. COVID-19 cases, health experts worry virus variants may erase pandemic progress.

Americans are being urged to get vaccinated as soon as possible, but the vaccine supply issue remains a big problem.

In the war on COVID-19, health experts agree it is one of the best defenses, but although more than 42 million doses have been given, in many places there still is not enough.

Faced with a vaccine shortage in California, Los Angeles County is only giving out second doses of the vaccine for the rest of the week, rather than offering any more first doses.

Pfizer says it has now increased production to double its coronavirus vaccine output.

The vaccine maker expects to deliver 200 million doses to the U.S. by the end of May.

Meanwhile, in the fight to slow the virus’ spread, Transportation Secretary Pete Buttigieg says the CDC is looking at all its options while considering whether passengers should be required to provide a negative COVID-19 test before domestic flights.

"What we know is that it's the appropriate measure for international travel, people traveling into the U.S., given some of those considerations. You know, I'd say the domestic picture is very different, but you know the CDC is always evaluating what can best be done to keep Americans safe," says Buttigieg. Doctors say the current vaccines are very effective against the U.K. variant, and it’s critical to get shots in arms now.