Although one dose of the vaccine will work against coronavirus, two doses are required for long-lasting and robust immunity from the virus.

Due to mutation, new variants of the virus such as the delta variant first identified in India keep emerging. Experts recommend getting fully vaccinated to boost the immune system against these variants.

The COVID-19 vaccines rolling out globally were developed to target the original version of the virus detected in late 2019. While they seem to work against newer versions, there’s a concern the shots eventually might lose their effectiveness if variants evolve enough.

With the delta variant, a study by British researchers found people were well protected when they got both doses of either the AstraZeneca or Pfizer-BioNTech vaccines. But with only one dose, protection was significantly reduced.

To stem the spread of the delta variant in the United Kingdom, British Prime Minister Boris Johnson recently delayed the lifting of remaining restrictions to get more people the full two doses.

Health officials are also concerned about the dozens of countries that still don’t have enough supply secured to distribute second doses within the recommended time frame.

World Health Organization (WHO) chief Tedros Adhanom Ghebreyesus has said vaccinations with only one dose won’t be enough to stop outbreaks fueled by new variants and that people should maintain social distancing and other measures until more of the population is fully vaccinated.

The second dose of a two-dose vaccine is critical because it’s what "really gives a boost to the immune system so that the antibody response is very strong," says Dr. Soumya Swaminathan, WHO’s chief scientist.