Are Covid Shots Working? What the Real World Tells Us

(Bloomberg) -- With Covid-19 shots in billions of arms by now, evidence is emerging of how well they work in real-world settings. Vaccines have successfully subdued epidemics in nations where at least half of the population is fully immunized. But as more-transmissible variants spread, driving fresh outbreaks and an increase in breakthrough infections, early signs suggest that vaccines lose some potency over time. Health authorities are recommending some immunized individuals wear face-masks indoors to augment the protection, and some are turning to booster shots as well.

1. How good are Covid vaccines?

In general, the Covid shots are extremely good, though not perfect, at protecting against hospitalization and death. Researchers in Taiwan found that a 10% increase in vaccine coverage is associated with a 7.6% reduction in the case fatality ratio. Most vaccines also provide a very good shield against developing Covid symptoms, and some are able to provide good protection from being infected with the virus that causes Covid, SARS-CoV-2. The latter ability is important because it results in the kind of sterilizing immunity that stops the virus from spreading -- a feat that will bring the world closer to ending the pandemic via so-called herd immunity. In any case, vaccination is associated with a 40%-to-50% reduction in Covid cases among an infected person’s household contacts, and it’s anticipated that newer generation vaccines will be even better at preventing onward transmission.

2. What are the caveats?

The evidence so far is skewed, with Israel, the U.S. and U.K. supplying the majority of the data, which pertains mostly to vaccines made by AstraZeneca Plc and Pfizer Inc. and its partner BioNTech SE. Estimates for Sputnik V and Sinopharm vaccines are the most limited. Also, although more than 90 vaccine effectiveness studies have been made publicly available, their quality varies considerably, according to the World Health Organization. Findings have to be interpreted carefully, especially when trying to compare studies, as variables such as who was observed, where and when can affect results. 

3. Are some vaccines better than others?

It seems so, though there have been no head-to-head clinical trials. For now, the so-called mRNA vaccines made by Moderna Inc. and by Pfizer-BioNTech appear superior overall to the other types. Moderna’s also appeared to be significantly better at preventing breakthrough infections than Pfizer’s, according to a review of people in the Mayo Clinic Health System in the U.S. from January to July. Also, mixing vaccines (most require two shots) appears to generate a robust immune response. Doctors have reiterated, however, that the best vaccine is the one that’s available to you, as any protection is better than none against this virus.

4. Why is it hard to tell?

Efficacy levels estimated from clinical trials aren’t necessarily comparable because each vaccine study used different regimens and measurements. They also were undertaken in different groups of people and at different times, when different strains were prevalent. In some countries, like Chile, two vaccines have been administered simultaneously, allowing for a comparison of the two groups. But the individuals receiving the shots may have differed in terms of their vulnerability and likelihood of being infected, so the effectiveness of each vaccine may not be directly comparable. Still, the results show how well vaccines are preventing Covid cases, hospitalizations and deaths from real-world data.

5. How durable is the protection?

We’ll know as more time goes by. An unpublished study by Pfizer researchers that followed vaccinated individuals for up to six months reported a gradually declining trend in vaccine efficacy against a symptomatic infection, but found their shot remained “highly efficacious” overall. Efficacy peaked at 96.2% in the two months after a second shot, and declined gradually to 83.7% from four months, and then by an average of 6% every two months. Waning vaccine-induced immunity is likely to be first detected by vaccine failures in vulnerable individuals, such as diabetes and heart-failure patients, and reflect lower levels of virus-blocking antibodies. U.K. researchers say that a likely scenario is that, as immunity gradually wanes, more vaccinated individuals will get infected, but their immune system will rapidly control the infection, resulting in no or mild symptoms. However, exposure to the virus will further boost and broaden their immune response to it. Vaccination campaigns against SARS-CoV-2 may continue for years.

6. What effect is delta having?

It’s the most worrisome strain detected so far. First reported in India in October, it’s been found in at least 163 countries. Observational studies have found vaccines show a diminished effectiveness at preventing new infections. Some researchers say that will require a higher proportion of people in the community to be immunized to slow, and eventually stop, transmission through herd immunity.

• The Centers for Disease Control and Prevention reported in August that vaccine effectiveness among front-line workers declined to 66% after the delta variant became dominant in the U.S., compared with 91% before it arose. It said the finding should be viewed with caution, however, because of the relatively short timespan covered and other complex variables.
• Another real-world study published in the same report showed vaccine protection against hospitalization was holding up well.
• Preliminary data from a U.K. study indicated that the effectiveness of both the Pfizer-BioNTech and AstraZeneca vaccines was reduced with the delta variant, and that the effectiveness appeared to decline at a faster rate over the first 90 days with the Pfizer shot, albeit from a higher starting point.
• Research out of Israel seems to back the idea that vaccine protection decreases in the months after inoculation, but it’s difficult to tell whether waning immunity to any type of coronavirus infection is to blame, or if the vaccinations are particularly ineffective against the delta variant, or both.
• Vaccines have been shown to reduce the concentration of virus particles, or viral loads, in the airways of people who become infected. But delta is associated with viral loads that are 1,200 times higher than the original strain -- a feature that may help delta spread as easily as chickenpox. That was seen in a dramatic outbreak in Provincetown, Massachusetts, in July, in which about three-quarters of the 469 Covid cases detected occurred in fully vaccinated people.
• Separately, the August U.K. study found that fully vaccinated people who became infected with delta had peak virus levels in the upper airways as high as those who’d had no shots.
• On the other hand, virus particles from individuals with delta are much less likely to be infectious if they’re emitted from fully vaccinated people, researchers in the Netherlands showed in a study released Aug. 21, ahead of publication.
• Immunized individuals are also infectious for a shorter period compared with those without immunity, reducing the likelihood of onward transmission, research from Singapore in July showed.

7. How do you fight it?

Some countries have turned to boosters even as many parts of the world still struggle to get any vaccines. Israel in late July became the first to approve the use of booster shots, for people over 60, and early signs are they seem to be working. U.S. health authorities are reviewing a Biden administration plan to administer boosters to the general population. The CDC has already recommended people return to wearing masks indoors in some situations. It considers delta more prone to breaking through vaccine-induced immunity and, in an internal document reported in July, argued officials must “acknowledge the war has changed.”

The Reference Shelf

• Related QuickTakes on coronavirus variants, including delta, vaccine nationalism, Covid vaccine reactions, breakthrough infections and mRNA vaccines.
• A list of Covid vaccines evaluated by the WHO, vaccine effectiveness studies, and the agency’s vaccine toolkit.
• The University of Melbourne’s weekly Covid vaccine updates.
• Natsuko Imai and Neil M. Ferguson interpret the estimates of Covid-19 vaccine efficacy and effectiveness.
• European epidemiologists assess vaccine effectiveness in seniors.

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