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COVID-19 Vaccines

Information and evidence about COVID-19 vaccine efficacy and safety

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CDC - Summary of Recent Changes

July 27, 2021

Data were added from studies published since the last update that demonstrate currently authorized mRNA vaccines provide protection against variants of concern, including the Delta strain that is now predominant in the United States. Vaccine effectiveness against hospitalization and death is high for all current SARS-CoV-2 variants; emerging data suggest lower effectiveness against confirmed infection and symptomatic disease caused by the Beta, Gamma, and Delta variants compared with the ancestral strain and the Alpha variant.

Key Points
  • All COVID-19 vaccines currently authorized in the United States are effective against COVID-19, including serious outcomes of severe disease, hospitalization, and death.
  • Available evidence suggests the currently authorized mRNA COVID-19 vaccines (Pfizer-BioNTech and Moderna) are highly effective against hospitalization and death for a variety of strains, including Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), and Delta (B.1.617.2); data suggest lower effectiveness against confirmed infection and symptomatic disease caused by the Beta, Gamma, and Delta variants compared with the ancestral strain and Alpha variant. Ongoing monitoring of vaccine effectiveness against variants is needed.
  • A growing body of evidence indicates that people fully vaccinated with an mRNA vaccine (Pfizer-BioNTech or Moderna) are less likely than unvaccinated persons to acquire SARS-CoV-2 or to transmit it to others. However, the risk for SARS-CoV-2 breakthrough infection in fully vaccinated people cannot be completely eliminated as long as there is continued community transmission of the virus.
  • Studies are underway to learn more about the effectiveness of Johnson & Johnson/Janssen vaccine.
  • At this time, there are limited data on vaccine effectiveness in people who are immunocompromised. People with immunocompromising conditions, including those taking immunosuppressive medications, should discuss the need for personal protective measures after vaccination with their healthcare provider.
  • This updated science brief synthesizes the scientific evidence supporting CDC’s guidance for fully vaccinated people and will continue to be updated as more information becomes available.

Interim Public Health Recommendations for Fully Vaccinated People

  • Updated information for fully vaccinated people given new evidence on the B.1.617.2 (Delta) variant currently circulating in the United States.
  • Added a recommendation for fully vaccinated people to wear a mask in public indoor settings in areas of substantial or high transmission.
  • Added information that fully vaccinated people might choose to wear a mask regardless of the level of transmission, particularly if they are immunocompromised or at increased risk for severe disease from COVID-19, or if they have someone in their household who is immunocompromised, at increased risk of severe disease or not fully vaccinated.
  • Added a recommendation for fully vaccinated people who have come into close contact with someone with suspected or confirmed COVID-19 to be tested 3-5 days after exposure, and to wear a mask in public indoor settings for 14 days or until they receive a negative test result.
  • CDC recommends universal indoor masking for all teachers, staff, students, and visitors to schools, regardless of vaccination status.



How do vaccines work?

Simply put, a vaccine trains the immune system to recognize viruses, bacteria, and other germs, or pathogens, so the body can react. The human body produces special proteins called antibodies to fight germs like the virus that causes COVID-19. Vaccines introduce weakened or killed parts of pathogens, or elements that mimic their structure, into the body.

COVID-19 vaccines cause your body to safely develop antibodies that recognize and fight the coronavirus that causes the disease, greatly reducing the risk for full-scale infection.

SOURCE: HHS Combat Covid

HHS - How do Vaccines Work?

How do mRNA Covid-19 vaccines (such as the Pfizer/BioNTech and Moderna Covid-19 vaccines) work?

Two messenger RNA (mRNA) vaccines are currently available in the United States, one developed by Pfizer/BioNTech (BNT162b2) and the other by Moderna (mRNA-1273). In these vaccines, the mRNA carries instructions to make the SARS-CoV-2 “spike” protein — the prickly projections on the surface of the virus, which is structured like those rubber balls that dogs love to chase or like the quills of a porcupine.

Once the vaccine is injected, the mRNA is taken up by the macrophages near the injection site and instructs those cells to make the spike protein. The spike protein then appears on the surface of the macrophages, inducing an immune response that mimics the way we fight off infections and protects us from natural infection with SARS-CoV-2. Enzymes in the body then degrade and dispose of the mRNA. No live virus is involved, and no genetic material enters the nucleus of the cells.

Although these are the first mRNA vaccines to be broadly tested and used in clinical practice, scientists have been working on mRNA vaccines for years. And despite this wonderful parody piece. opens in new tab saying that the technology is “obvious,” in fact the breakthrough insight that put the mRNA inside a lipid coating to prevent it from degrading is quite brilliant — and yes, this may be the first time the New England Journal of Medicine has referenced a piece in The Onion. (Last reviewed/updated 23 Mar 2021)

SOURCE: NEJM Covid-19 Vaccine - FAQs


How do adenovirus Covid-19 vaccines (such as the Johnson & Johnson vaccine) work?

The J&J vaccine is a recombinant replication-incompetent human adenovirus serotype 26 vector encoding a full-length, stabilized SARS-CoV-2 spike protein antigen. That’s quite a lot to digest, so let’s unpack it in words that are more easily understandable by us mere mortals:

  • “recombinant” — genetically engineered in a lab. Yes, I know it can mean more than this, but that’s the gist.
  • “replication-incompetent” — can’t reproduce in humans. No risk of viral dissemination.
  • “adenovirus serotype 26” — there are many adenovirus serotypes in the community. This one is uncommon, which is important because preexisting immunity to an adenovirus might diminish our response to the vaccine. The name of the J&J vaccine refers to this serotype (Ad26) and the antigen it carries — Ad26.COV2.S.
  • “vector encoding” — the virus acts as a transporter (vector) of the gene — the DNA — that makes (encodes) the antigen, which is a …
  • “full-length, stabilized SARS-CoV-2 spike protein antigen” — the researchers have taken the genetic machinery, or DNA, from SARS-CoV-2 that makes the whole spike antigen, and made it stable enough to be carried within the adenovirus.

When the vaccine is injected, the adenovirus cannot replicate, but its genetic material can enter the host cell’s nucleus and be converted to messenger RNA. The mRNA then creates spike protein antigens from SARS-CoV-2, which elicit a host response similar to natural infection with the virus that causes Covid-19. Both antibody and cellular immunity is stimulated.

Note that there are other adenovirus vector vaccines in use globally that carry the SARS-CoV-2 spike protein DNA. Neither is yet available here in the United States, and both use two doses as their vaccine strategy. The Oxford/AstraZeneca vaccine uses a chimpanzee adenovirus modified so it does not replicate in humans. The Sputnik V vaccine starts with the same Ad26 adenovirus as the J&J vaccine, but then switches to adenovirus serotype 5 for the second dose. This switch theoretically improves the boosting effect of this vaccine, as it reduces the chance that acquired immunity to Ad26 will diminish the immune response. (Last reviewed/updated 29 Mar 2021)

SOURCE: NEJM Covid-19 Vaccine - FAQs


The People's Pharmacy Radio Program

Show 1256: How Vaccine Diplomacy Can Help Prevent the Next Pandemic

What Is Vaccine Diplomacy? When there is a world-wide pandemic, pathogens don't stop at national borders. Vaccines should not, either. Dr. Hotez describes a remarkable incident in which the US and the USSR collaborated on the development of the oral polio vaccine in the middle of the Cold War. Simply developing vaccines is not enough, however. People must also embrace the opportunity to be vaccinated. Suspicion in a few areas has slowed the uptake of polio vaccine and delayed eradication of the disease. Simply developing vaccines is not enough, however. People must also embrace the opportunity to be vaccinated. Suspicion in a few areas has slowed the uptake of polio vaccine and delayed eradication of the disease.