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New data finds COVID-19 cases are dropping in some vaccinated groups. JACK GUEZ/Getty Images
  • As more people are vaccinated against the coronavirus, researchers will be able to see how well the vaccines work in the real world.
  • Real-world effectiveness of vaccines is often lower than the efficacy seen in clinical trials due to a number of factors.
  • Recent data from California and Israel find that cases are dropping as more people are getting vaccinated.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date. Visit our coronavirus hub and follow our live updates page for the most recent information on the COVID-19 pandemic.

Last week the Los Angeles Fire Department reported a dramatic decrease in daily COVID-19 cases since firefighters started receiving the coronavirus vaccine in late December.

With around three-fourths of the city’s firefighter department members vaccinated, a memo from Fire Chief Ralph Terrazas reported that the department’s daily number of positive tests dropped from nearly 20 in December to less than 5 last week. The memo was originally obtained by the Los Angeles Times.

Experts say this is a sign that the coronavirus vaccines — which showed high efficacy in late-stage clinical trials — are also working in the real world.

So is the United States’ rollout of the vaccine also behind the declines in COVID-19 cases and hospitalizations seen last week in almost every state?

Experts say it’s unlikely, given that so far only 9.6 percent of Americans have gotten at least one vaccine dose.

“It is impossible to know the causes of the decline in cases and hospitalizations, given that [so few] of the population has been vaccinated,” said Dr. Sadiya S. Khan, an assistant professor of medicine and preventive medicine at Northwestern University Feinberg School of Medicine.

Other experts say these recent declines in cases are likely due to changes in people’s behavior — such as traveling and gathering less after the holidays, or wearing masks and physical distancing more in response to surging cases and news of hospital bed shortages.

But at some point, when enough people have been vaccinated, the country will start to see positive effects of the vaccine on the pandemic.

“If we are aggressive in rolling out the vaccine effectively, we can continue this downward trajectory,” Khan said. “This is especially critical right now as new variants that are more contagious are becoming more prevalent and could send us backwards, as we are seeing in Europe.”

Israel, which is leading the world in coronavirus vaccinations, is starting to see benefits from its rapid rollout of the vaccine.

Currently, 57.6 percent of its population has been vaccinated, according to the non-profit organization Our World in Data. The vaccination rate is even higher among Israelis ages 60 or older, who were among the first in the country to receive the vaccine.

Last month Clalit, Israel’s largest health service organization, released preliminary data on 200,000 people 60 years or older who were vaccinated, comparing them to 200,000 similar unvaccinated older adults.

The positivity rate dropped 33 percent among those who were vaccinated, 14 days after they received the vaccine. No decline was seen in the unvaccinated.

Maccabi, another Israel healthcare organization, saw an even larger drop, reports The New York Times. Infections decreased by 60 percent among 430,000 people, 13 to 21 days after they received the vaccine.

The data is preliminary and has yet to be published in a peer-reviewed journal, so it should be viewed with some caution.

But Israel’s success offers early clues to what other countries might experience as they ramp up their vaccination efforts.

“It looks like the vaccine in Israel has been really successful,” said Christina Ramirez, PhD, a professor of biostatistics at the UCLA Fielding School of Public Health. “And I think it’ll tell us what we should expect in other countries.”

Late-stage clinical trials have already shown that the Pfizer-BioNTech vaccine has an efficacy of 95 percent against symptomatic infection, and the Moderna-NIAID has an efficacy of 94.1 percent.

Interim data also shows that the Johnson & Johnson vaccine has an efficacy of 66 percent against moderate to severe disease. This study did not look at whether the vaccine prevented mild cases.

Real-world effectiveness of vaccines is often lower than the efficacy seen in clinical trials due to a number of factors.

Israeli researchers plan on doing more in-depth statistical analysis of the data, reports the New York Times. They will likely try to take into account some of these other factors.

In spite of the effectiveness of the coronavirus vaccines and Israel’s fast pace in getting doses into people’s arms, the country is being hit hard by a third wave of the coronavirus.

The emergence of more infectious variants is being blamed for the inability of the vaccination campaign and a lockdown to slow the spread of the virus, according to the health minister in Israel.

This includes variants first identified in the United Kingdom and in South Africa, both of which spread more easily.

Some research suggests the vaccines may be less effective against the variants, in terms of preventing all symptoms. But they may still be able to prevent severe cases, hospitalizations, and deaths.

This was seen with the interim data from the Johnson & Johnson clinical trial. The vaccine had a lower efficacy overall in South Africa, but it still had a high efficacy against severe disease in that country.

Research on these variants is ongoing, as is monitoring for the spread of these variants and the emergence of new variants.

Ramirez says this data is crucial for our fight against the coronavirus.

“We need to be sequencing and keeping track of these mutations,” she said, as well as looking at whether the vaccines prevent transmission of the virus from one person to another.

“Right now we’re saying that you need to wear masks and socially distance even if you’re vaccinated,” she said.

But if the vaccines also keep people from spreading the virus, then some of those public health measures could be gradually relaxed.

“This is an important question to address, because that will help us get back to normal faster,” Ramirez said.

Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, says rapid rollout of the vaccines could also help slow the emergence of new variants in the first place.

“Viruses cannot mutate if they can’t replicate,” Fauci said Monday at the White House’s COVID-19 response team press conference. “If you stop their replication by vaccinating widely… not only are you going to protect individuals from getting disease, but you are going to prevent the emergence of variants.”

So when will Americans start to see the benefits of the vaccine rollout?

With the sluggish vaccine rollout that has resulted in less than 2 percent of Americans being fully vaccinated, it could take a few months before the drop in cases and hospitalizations can be tied to the vaccine. By then, millions more Americans will have been vaccinated.

Teasing apart the effects of the vaccine from those of other measures like masking, physical distancing, and lockdowns hinges on having enough fine-grained data to do a thorough analysis.

However, gathering consistent data from state to state is something the United States has struggled with throughout the pandemic.

“In the data that I’ve seen, the [Centers for Disease Control and Prevention] is not keeping track of people who become infected after their first dose or after their second dose of the vaccine,” said Ramirez.

“That’s the type of data that we would need to be able to say: This decrease is due to the vaccine versus this decrease is due to other policy measures.”