[11] Immunity from previous recovery[12][13] or COVID-19 vaccines are effective in preventing severe disease or hospitalisation from infection with the variant.

[25][24] By late July, it had also driven an increase in daily infections in parts of Asia,[26] the United States,[27] Australia, and New Zealand.

[34][35] Simultaneously, the ECDC released a brief maintaining all three sublineages of B.1.617 as VOI, estimating that a "greater understanding of the risks related to these B.1.617 lineages is needed before any modification of current measures can be considered".

Mid-October 2021, AY.4.2 accounted for an estimated 10% of cases, and has led to an additional growth rate rising to about 1% (10% of 10%) per generational time of five days or so.

The Indian Council of Medical Research (ICMR) found that convalescent sera of the COVID-19 cases and recipients of Bharat Biotech's BBV152 (Covaxin) were able to neutralise VUI B.1.617 although with a lower efficacy.

[68] Anurag Agrawal, the director of the Institute of Genomics and Integrative Biology (IGIB), said the study on the effectiveness of the available vaccines on lineage B.1.617 suggests that post-vaccination, the infections are milder.

[69] Anthony Fauci, the Chief Medical Advisor to the President of the United States, has also expressed his confidence regarding the preliminary results.

[70]Another study by the Centre for Cellular and Molecular Biology (CCMB) in Hyderabad found Covishield (Oxford–AstraZeneca) vaccinated sera offers protection against lineage B.1.617.

[71] A study conducted by Public Health England (PHE), found that compared to those who were unvaccinated those who were vaccinated with either the Pfizer-BioNTech or AstraZeneca-Oxford had 33% less instances of symptomatic disease caused by the variant after the first dose.

Among those who were two weeks after the receiving their second dose of the Pfizer-BioNTech vaccine 88% less subjects had symptomatic disease from the Delta variant versus those that were unvaccinated.

Among those who were two weeks after the receiving their second dose of the AstraZeneca-Oxford vaccine 60% less subjects had symptomatic disease from the Delta variant versus those that were unvaccinated.

[76][77] On July 3, researchers from the universities of Toronto and Ottawa in Ontario, Canada, released a preprint study suggesting that the Moderna vaccine may be effective against death or hospitalization from the Delta variant.

[83] On August 10, a study showed that the full vaccination coverage rate is correlated inversely to the SARS-CoV-2 delta variant mutation frequency in 16 countries (R-squared=0.878).

[90][91] These basic reproduction numbers can be compared to MERS (0.29-0.80[92]), seasonal influenza (1.2–1.4[93]), Ebola (1.4–1.8[94]), common cold (2–3[95]), SARS (2–4[96]), smallpox (3.5–6[97]), and chickenpox (10–12[98]).

[116] In the Netherlands, the virus was still able to propagate significantly in the population with over 93.4% of blood donors being tested positive for SARS-CoV-2 antibodies after week 28, 2021.

[117][118] Similar high seroimmunity levels occur in the United Kingdom in blood donors and general surveillance.

[122][123] Preliminary data from a study with 100,000 volunteers in the UK from May to July 2021, when Delta was spreading rapidly, indicates that vaccinated people who test positive for COVID-19, including asymptomatic cases, have a lower viral load in average.

[129] On June 14, researchers from Public Health Scotland found that the risk of hospitalization from Delta was roughly double that of from Alpha.

"[132] James McCreadie, a spokesperson for Public Health England, clarified "It is too early to assess the case fatality ratio compared to other variants.

is more serious and resulted in an increased risk of death compared to previous variants, odds that are significantly decreased with immunization.

[204] The variant has been identified as a super-spreader and has led to the lockdowns of five cities (Lautoka, Nadi, Suva, Lami and Nausori), an area which accounts for almost two-thirds of the country's population.

[166] The Philippines confirmed its first two cases of the variant on 11 May 2021, despite the imposed travel ban of the country from the nations in the Indian subcontinent (except for Bhutan and Maldives).

[206] The detection of B.1.617 was hampered in some countries by a lack of specialised kits for the variant and laboratories that can perform the genetic test.

[207][208] For example, as of 18 May, Pakistan had not reported any cases, but authorities noted that 15% of COVID-19 samples in the country were of an "unknown variant"; they could not say if it was B.1.617 because they were unable to test for it.

[213][214] After the rise in cases from the second wave, at least 20 countries imposed travel bans and restrictions on passengers from India in April and May.

[215][216][217] In May 2021, residents of two tower blocks in Velbert, Germany, were quarantined after a woman in the building tested positive for the Delta variant.

[218] In May, Delhi Chief Minister Arvind Kejriwal said that a new coronavirus variant from Singapore was extremely dangerous for children and could result in a third wave in India.

[220] On 23 June, the province of Ontario in Canada accelerated 2nd dose vaccine appointments for people living in Delta hot spots such as Toronto, Peel and Hamilton.

[226] On 9 July, Seoul, South Korea ramped up restrictions urging people to wear masks outdoors, and limiting the size of gatherings.

The government cited the protection and wide coverage of the COVID-19 vaccination programme, although health experts expressed concern at the move.