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Early studies indicate boosters can increase the effectiveness of COVID-19 vaccines against Omicron. Joe Raedle/Getty Images
  • A two-shot course of mRNA vaccines or the one-shot J&J vaccine seem to be less effective against the Omicron variant, especially for infection.
  • Data so far indicates that mRNA vaccines (Pfizer-BioNTech or Moderna) offer the most promising protection against both infection and hospitalization, in line with the CDC’s recommendations.
  • Current figures suggest that vaccines offer 30 to 40 percent protection against infection and around 70 percent protection against hospitalization without boosters.
  • Newer data is confirming that a third dose increases antibody production and boosts effectiveness against infection to around 75 percent, and 88 percent for severe disease.

This is a developing story. We’ll update it as we learn more.

Omicron is spreading rapidly across the globe, and researchers are racing to understand how vaccines will hold up against this new variant of the coronavirus.

Several preliminary studies have assessed the effectiveness of current COVID-19 vaccines in use against the Omicron variant.

So far, one- or two-dose vaccines provide far less protection than those paired with a booster, but they still do appear to protect against severe disease.

Studies conducted in the lab and the real world show that full vaccination plus a booster shot provide stronger protection against infection with Omicron.

It is important to remember most of these are lab studies and may not reflect the vaccines’ real-world performance.

Here is what we know so far.

Early estimates suggest that vaccine effectiveness against symptomatic infection with Omicron is significantly lower compared with the Delta variant.

A report by Imperial College London indicated that the risk of reinfection with Omicron was 5.4 times greater than the Delta variant. Previously having COVID-19 also afforded little protection against reinfection with Omicron.

“This is such a contagious virus that it can spread not only among the unvaccinated, where I think it still has a very substantial risk of causing serious disease that might require hospitalization, but it can also spread among vaccinated persons, although the illness it produces among the vaccinated, particularly if you’ve had a boost, is generally mild and even without symptoms,” said Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center in Tennessee.

However, the Imperial College London study suggests there is “no evidence of Omicron having different severity from Delta,” despite hospitalization remaining relatively low for the time being.

“[Even if] Omicron may not lead to more severe illness than Delta, a rapid and massive surge in infections could still overwhelm hospitals with sick patients. People who are unvaccinated remain at the highest risk, but also those who have not received a third dose of an mRNA vaccine,” said Dr. Robert Glatter, an emergency physician at Lenox Hill Hospital in New York City.

A new report from the Centers for Disease Control and Prevention (CDC) found that mRNA vaccines were still highly effective at preventing serious disease and death during the Omicron wave.

The report published Friday in the CDC’s Morbidity and Mortality Weekly Report looked at data from over 7,000 patients, including over 1,400 who ended up on a ventilator.

People who had at least two doses of either Pfizer or Moderna COVID-19 vaccines were much less likely to need a ventilator after developing COVID-19 and less likely to die from the disease.

They found that people who had at least two doses of an mRNA COVID-19 vaccine had a 90 percent reduced risk of being put on a ventilator or dying from the disease. During the Omicron wave, people who had three doses had a 94 percent reduced risk of being put on a ventilator or dying from COVID-19.

Few real-world studies have managed to estimate effectiveness for the Pfizer-BioNTech vaccine, and several lab studies show that a booster increases antibody production significantly.

“The studies are largely laboratory studies that would indicate that levels of antibody induced by boosting will have a noteworthy impact on preventing serious disease,” said Schaffner.

A preprint study conducted by Oxford University reported that two doses of the AstraZeneca or Pfizer-BioNTech vaccines offered little protection against infection with the Omicron variant.

However, a real-life study from South Africa found that two doses of the Pfizer-BioNTech vaccine still protected people from severe disease.

Researchers found that two doses provided 70 percent protection against hospitalization and 33 percent protection against infection. This was a drop from about 93 percent and 80 percent, respectively, for the Delta variant.

A study published in the New England Journal of Medicine echoed these figures, finding that a two-dose regimen of the Pfizer-BioNTech vaccine was 70 percent effective against hospitalization with Omicron.

Dr. Monica Gandhi, an infectious diseases specialist with the University of California, San Francisco, told Healthline that although these lab studies show that antibodies, the body’s main line of defense against the coronavirus, can wane over time or be affected by mutations along with the virus’s spike protein, vaccines were able to generate other types of immune cells to fight off infection.

“We know now that T cells from the vaccines still work against Omicron, and B cells generated by the vaccines adapt the new antibodies they produce to work against variants. So, I think the clinical scenarios we are seeing in those with prior immunity, vaccinated, and even boosted individuals [show that] the vaccines are likely still protecting Omicron-infected individuals from severe disease,” she said.

Initial studies suggest that two doses of the Pfizer-BioNTech COVID-19 vaccine offer 30 to 40 percent protection against infection and around 70 percent protection against hospitalization.

Early analysis from the United Kingdom has suggested that boosters have moderate to high vaccine effectiveness against symptomatic infection, offering 70 to 75 percent protection in the initial weeks after the booster.

Researchers found a slight difference depending on whether the initial doses were from the AstraZeneca vaccine or Pfizer-BioNTech vaccine. Both groups had Pfizer-BioNTech boosters.

Two weeks after a Pfizer-BioNTech booster dose, vaccine effectiveness for people who received the AstraZeneca vaccine increased to around 71 percent and about 76 percent for those who initially got the Pfizer-BioNTech vaccine.

As for severe disease, modeling by Imperial College London found that the Pfizer-BioNTech vaccine with a booster was around 85.9 percent effective against Omicron, compared with about 97 percent against Delta.

Early data from Pfizer had indicated that a booster dose significantly increased neutralizing antibodies, bringing the vaccine’s protection close to what two doses provided against the original variant of the coronavirus.

The data comes from a series of lab experiments testing the neutralizing ability of blood samples from people who had two doses of the vaccine and those who had received a booster.

The neutralizing ability of the antibodies collected from those who did not have a booster had a more than 25-fold decline against Omicron than the original variant.

Similarly, a study in Israel comparing blood samples from two groups of healthcare workers who had or had not received Pfizer-BioNTech boosters found that the third dose increased antibody levels 100 times compared with two doses alone.

The most recent report by the U.K. Health Security Agency (UKSHA) showed that a third dose offers significantly higher protection.

Early results showed that vaccine effectiveness dropped to 52 percent against Omicron 6 months after the second dose, while an mRNA booster brought this up to 88 percent against hospitalization.

A study recently published in the journal Lancet also found that people who had received the Pfizer COVID-19 vaccine for all three doses had significantly high neutralizing antibody levels against Omicron. They saw that it was similar to the level provided by two doses against Delta.

Antibody levels overall were almost 2.5 times higher against Omicron after three doses compared to two.

The researchers noted that antibody levels alone cannot predict vaccine effectiveness but are a good indicator of protection against severe COVID-19.

With a booster, the Pfizer-BioNTech COVID-19 vaccine’s protection against infection rises to around 75 percent, and 80 to 90 percent for severe disease. These numbers may change as more research results come in.

No studies offer definitive estimates for the Moderna vaccine’s effectiveness against Omicron, though scientists believe it may be similar to Pfizer-BioNTech’s vaccine.

A preliminary study of blood samples from 30 people who had received two doses of the Moderna vaccine showed that their antibodies were at least about 50 times less effective at neutralizing Omicron.

With a 50-microgram booster, however, antibodies increased 37-fold. On the other hand, a 100-microgram booster, the amount given to immunocompromised people as a third primary shot, increased antibody levels more than 80 times.

A UKHSA analysis of nearly 200,000 cases showed that vaccine effectiveness of three doses offered 88 percent protection against hospitalization with the Moderna or Pfizer-BioNTech vaccines.

Compared with about 72 percent effectiveness provided by two doses, a three-dose course of the Moderna vaccine is estimated to be 88 percent effective against hospitalization. More data is needed to assess its performance against infection with Omicron.

A Centers for Disease Control and Prevention (CDC) panel in December 2020 recommended that people get the Pfizer-BioNTech or Moderna vaccine over the Johnson & Johnson vaccine due to the rare risk of blood clots.

Furthermore, data from South Africa showed that the J&J vaccine produced virtually no antibodies against Omicron in lab experiments.

“Those who received the J&J vaccine (in the past few months) and are worried are past the period of concern. But if they’ve just received one dose of J&J, they should be urged to get a booster,” said Schaffner, commenting on the latest data.

Although it is not yet a formal recommendation, mixing vaccines has been shown to increase immunogenicity, which is why physicians are advising that booster doses for J&J be Moderna or Pfizer-BioNTech.

“They (those who received J&J vaccines) will get a much higher antibody level [with mRNA boosters],” said Schaffner.

Gandhi, meanwhile, said that although she generally agreed with the CDC panel recommendation, there were still groups of people who may benefit from this vaccine.

There is a lack of data to determine J&J vaccine effectiveness. However, the single-shot vaccine seems to perform poorly in terms of antibodies against Omicron.

Glatter said that studies so far demonstrate that people who are fully vaccinated and boosted retain the highest and most durable level of protection against severe disease and hospitalization with the Omicron variant.

However, he stressed that the Omicron variant can evade the protection of vaccines to a degree, “especially in causing infection in people who have not received a booster.”

“This translates to more breakthrough infections, some of which are mild but more serious in those who have organ transplants, are immunocompromised, or are on chemotherapy. [T]he evidence indicates that boosters can restore protection, even against infection,” Glatter said.

Schaffner also said he agreed with studies showing that two doses did not offer enough protection against Omicron infection.

“Certainly here in the [United] States, we are strongly recommending that people who’ve had two doses of the mRNA vaccines receive a booster just as quickly as possible,” Schaffner said.

“We continue to urge those who have not yet been vaccinated to initiate their vaccination series, and to bring their young children along with them because here in the U.S., every child aged 5 and older now is eligible for vaccination,” he added.